If there is a document for historians to consult, it means someone had to preserve it, and it had to be accessible. So a childhood diary for most people would not be preserved. But someone who becomes perceived of as an "important historical figure," their relatives, governments, organizations, etc., might make an effort to both save such things (unlike my parents, who have started off-loading "junk I made as a child" onto me, as if I want it, in order to free up some storage space!) and to potentially make them accessible.

It is worth talking about the difference between preservation and accessibility. Sometimes things are preserved but are not accessible: e.g., they are kept in private hands, such as in the hands of families or collectors. That means that if a historian wants to use them, they must a) figure out they exist, and b) somehow convince the people who have them to give them access. Neither of these are inherently easy. If a previous historian has not used a source, there may be no record that it exists — so it will take a lot of sleuthing or luck to even find out if said source exists. Even if they know about a source, convincing whomever controls it privately to let the historian have access can be very tricky, especially if it is family — they person or group who owns it may put conditions on is use, may ask for veto power over the final publication, may simply deny use of it. Many, many historians have stories about how they got some private source to give them access to something (a friend of mine used Carl Sagan's papers before they were put into the Library of Congress, and that meant going through Sagan's widow, who was extremely concerned with his positive scientific legacy, and so put very difficult requirements on their use in any publication — like requiring veto power over all quotes used; a mentor of mine, a long time ago, had to basically wine-and-dine his way into the noble French household that now had Lavoisier's papers).

A much, much easier approach is if this material is deposited into archives. Archives are basically organizations that hold, catalogue, and make accessible (to various degrees) historical records. They get their records in various ways, but usually they are donated (by the private parties who had them originally), deposited by governments (e.g., in the US, federal records are mandated by law to eventually be processed for archival deposition), or purchased (from private holders, or other archives). The archivists work to acquire the records (and they don't try to acquire everything — they have limited space and bandwidth — so they try to be selective), then process/catalogue them (making Finding Aids that allow people to know what records they have, which is a huge amount of work, and if done well dramatically increases accessibility, and if done poorly dramatically hinders it), and then make them to some degree accessible (archives sometimes have barriers to access, especially for individual records, which might, say, be classified, or sealed for privacy reasons for some number of years, and so on).

There is no single archive for everything. There are multitudes of archives. Some are private, many are not-for-profit, many are governmental. A historian working on a given topic spends a lot of time learning where the relevant archives and records are, and then going through Finding Aids and talking to archivists to figure out what they have and what they don't. Most of this work is in reading the footnotes of other people in the field, and seeing what kinds of records they are working with — that is usually the easiest way to locate the already-known relevant archives.

There are also some resources that try to aggregate archival information on specific topics. For example, the Niels Bohr Library at the American Institute of Physics maintains a database called the International Catalog of Sources for the history of physics, and it basically tries to keep track of where in the world there are archival collections of relevance to the history of physics. So this is sort of a meta-Finding Aid that contains information about lots of places. Pre-Internet there were more of these sorts of projects than there are now, I believe, because it was much harder to know where things might be otherwise.

Today a lot of this stuff is online, so historians spend a lot of time using Google and other search engines to locate collections and Finding Aids. Some of this can be very straightforward, a lot of it is not, because Google does not always index database results. So one tries to find relevant archives and then run search queries through them until you find something promising. Again, this depends on how detailed, complete, and well-done the Finding Aid data is. A LOT of archival data is not online; for example, the National Archives and Records Administration Archives II facility (the main archive for US government records) has only very "high level" information about most of its holdings online, e.g., a collection title with some keywords. Whereas in person, at the main Finding Aid room in Archives II (in College Park, MD), there are binders and binders full of detailed descriptions of box titles, folder titles, even record titles. But you have to go in person and use the binders, so that's a huge barrier of entry. (One thing I have done is copy a bunch of the most relevant Finding Aids for my research, so I can peruse them without being there physically, and share them with other researchers. It saves a huge amount of time and money and effort to do things this way.)

Anyway — historians rarely work to "find" genuinely new sources themselves. Sometimes we do. But usually we use things that have, through one route or another, have made their way into archives. Sometimes, we are part of that process (we might convince someone to donate their papers, for example, if we find out they have historically significant documents). But most of the time we are looking at things that have already been catalogued, and often already cited. (The latter cannot really be overstated — a good number of documents we use are documents another historian has already used and cited, drawing our attention to it.)

As for what kinds of documents literally survive, it depends on the medium, the conditions it went through historically (the Dead Sea Scrolls are unusual in that they were stored in very good conditions for long-term preservation of their medium — they are certainly very unusual and rare documents!), and the later work of preservation by archivists (which is part of their job as well).

Anyway, the long and short of it is that learning how to figure out where the relevant sources are, and getting access to them, is a LARGE part of what it means to be a historian. From the outside it looks impossible, and it certainly is a bear when you are starting totally from scratch, but it is something one can learn how to do, and the more you stick at it, the more resources one has for doing it.

u/restricteddata gave you an excellent answer about how historians get access to archived and preserved information in general. Unfortunately, I'm not the right person to go into more detail about the Himmler diaries that prompted your question other than to say that many people saved their filled diaries in the 19th-early 20th centuries. I can also point you to this paper which discusses some of how they fell into allied hands because I was curious about this as well.

However, I can comment on one of your other questions.

And this all goes of course for even older sources before the printing press? How do historians have all of this information, because I can't imagine that paper documents have survived hundreds, or sometimes even thousands of years. I mean sure there are the dead sea scrolls, but that is more the exception rather than the rule ?

As restricteddata said, it depends a lot on the medium, and you've both correctly noted that Dead Sea scrolls are a rare example of the environment assisting in preservation. That can often be the key factor in preservation. The type of writing, and whether anyone cares to preserve it, also matters. Finally, age is an obviously important factor. A piece of writing from 1400 CE has a much better chance of surviving than an identical piece from the same place in 1400 BCE.

Starting with medium, this may matter more than anything else. There's no single type of paper, and there are many non-papers that are used similar to the modern writing material made from plant fibers. Typically, paper today is meant to be mass produced and cheap, so we choose material that fits that need. Early paper was a little bit more durable because it was often made from recycling scraps of fabric to create so-called "rag paper." That originated in China in the 2nd Century CE and reached Europe by about 1000. It's thicker, more fibrous, and more water resistant even if the ink used on it is not. The down side is that it doesn't fold as nicely over time, so it becomes easy to tear. Still, it's just cotton and that won't last forever.

A little bit better than that is parchment (and its more expensive, fancier cousin, velum). Rather than being made from plant material like true paper, parchment is actually made from scraped and chemically treated animal hide. As a result, it retains some of dried skins' durability. That was actually one of the reasons British Parliament maintained official records on velum until 2017. The downside is that because of how it's treated, parchment doesn't handle water or humidity well. Without air conditioning, that basically dooms it unless its intentionally maintained.

Parchment evolved out of writing on leather, which is more durable but harder to write on. That transitioned happened so long ago (around the 3rd Century BCE in Europe) that even leather doesn't really stand the test of time all that well. It also was just never quite as popular as papyrus, which is similar to paper because its plant fiber but different because its woven instead of milled. Papyrus becomes very dry and very brittle quickly, but was a relatively cheap medium in ancient times, so much so that the papyrus reed used to make it spread from India to Morocco and was over cultivated to near extinction anyway.

But even before that, writing was done on clay and in stone. Wet clay is obviously impermanent, but that could be fired and hardened for long term storage. Both fired clay and stone carvings will last basically forever unless they're smashed by something. In ancient Egypt and the Near East, clay tablets were the first widespread writing medium, which is why comparatively many records have survived from those cultures than many more recent examples.

Environment is a major factor as well. Eventually, the regions that had used clay transitioned to leather and papyrus. Egypt made that leap pretty early on since papyrus was native to the Nile. The problem is that on a long enough timeline, humidity, the sun, and decomposers will take their toll on most organic material. Even leather rots away when exposed to the elements for years. But extremely dry environments both prevent water damage and become inhospitable to most of the microbes and animals that would break down the more paper-like media. Egypt, the salt caves of the dead sea, and other deserts can preserve perishable material for millennia because the sterile conditions.

Especially in a sandy environment, abandoned structures just get buried over time. In a more humid environment, this means moist soil full of worms and bacteria that rot away whatever you leave there. In the Sahara, it's a preservative. The same thing happens even faster to areas we don't care about or burry intentionally, like garbage dumps or graves. This is why so many caches of documents from about 5000 years of history and many different cultures have been found in Egypt. Even inside a building in the same environment, preservation is much easier.

Human care is the determining factor when the environment is inhospitable. The vast majority of documents we have today from the ancient and early medieval worlds are not original copies. Most of them aren't even from any time close to when they were first written, with the exception of some immensely popular works like the Iliad or Bible. They are copies, of copies, of copies, of copies.

People knew that their documents were impermanent, so if they wanted to keep them or share them, they had to make another example of the same document. Sometimes, for official records, multiple copies would be made for multiple locations right away. Medieval monks and ancient librarians did the grunt work of copying anything they thought was worthwhile for centuries, but as a result what survives is a reflection of what all those people thought was worthwhile. Writing that was considered a great example of its language, favorite stories and poems, or useful histories were all preserved. Business or government records, bad writing, and historical documents that someone felt were redundant weren't so fortunate. There are many lost historical authors known to us now primarily because they were cited by someone whose work has actually survived.

All of this writing was stored in the pre-modern equivalents of archives, libraries, and private collections that were all susceptible to similar problems. They could catch fire, be flooded, be abandoned in a time of emergency, or be disregarded by later generations. Often, a business or government's archives were only maintained so long as that entity functioned. If someone else took over, the archive was abandoned or even destroyed. Individual works, or bodies of work, could fall out of favor or use because of social circumstance, be that political propaganda against them or a changing religious vision of what people should be interested in.

As I said before, age is pretty obvious, especially as it relates to environment. More time exposed to the elements is more time to become damaged. Even in the Egyptian sands, course material shifting over time and lack of care eventually wares away large sections of brittle material. A salt cave is a rare thing in that regard, and even some of the Dead Sea Scrolls got wet enough that the rolled up layers of scrolls started fusing together.

The age of a document also had an effect on the language and culture of its readers, and whether or not they still cared. For example Old English documents were maintained in the same monasteries for generations, but eventually Middle English speaking monks started looking at them and couldn't even understand what they were copying any more. In Iran, the 6th Century Middle Persian Khwaday-Namag was a history of the real and legendary kings of Persia, and was even used as a source when Ferdowsi wrote his epic Shahnameh in the 11th Century, covering the same topic in the New Persian language with a more Islamic lens. Today, the Khwaday-Namag is completely lost, but the Shahnameh is the Iranian national epic, in large part because it is so much more relevant and understandable.

All of this can be easily observed on smaller and less impactful scales in modern day-to-day life to. A particularly treasured birthday card on heavy card stock kept in box and rarely handled, or a collectors edition of a favorite book on a shelf, will last significantly longer than a receipt for groceries that's mostly a formality anyway. Every time and place has their rough equivalents that receive similar treatment.

While both answers in the thread are fabulous, I want to further draw out the role of the archivist in doing a lot of the heavy lifting of preserving past writings.

Starting in the 18th century in France and the United States, but really not picking up until the 19th, there starts to be an awareness that the preservation of ephemeral documents (i.e. communications, government reports, etc.) were something that ought to be preserved, and there ought to be people who tried to preserve them. The field developed by the early 20th century into something separate from and distinct to the historian's work. To paraphrase an early leader in the field, Sir Hilary Jenkinson, the archivist's job was to preserve things as closely as possible to the way that they were produced, while the historian's job was to take those preserved materials and interpret them.

Jenkinson's framework is no longer used by archivists, but it was influential, and by about 1930 (when the US National Archives was established), every European and North American country had an archival system in place for government records. What this actually means is that it is illegal to destroy government records for a certain length of time after they are created, and they must be turned over, intact and in the order they were produced, to the archives who will process them and remove anything irrelevant, illegible, or duplicated.

This was actually true for the nazis as well - they had enormous government archives that tracked the correspondences and reports of most officials. While they tried to destroy their own archives at the end of WW2 to destroy evidence of the Holocaust and other crimes against humanity, the majority of the archives were salvaged by Allied forces and preserved. They were originally separated among institutions in Allied countries, but they have mostly recongregated in Stanford University, which has the finding aid.

There are a vast number of people involved in archival preservation and processing work, not to mention reprocessing the collections as material is traded between universities or donated. That same field both acquires records, helps stabilize them so they don't disintegrate, and makes them accessible so that historians are able to use them without further damaging the materials and the contexts the materials surround in.

This idea holds true for almost any piece of primary evidence! If you ever wonder "how did something survive this long" there is almost always archivists (or special collections conservators) to thank for putting in that work.