The first category are primary sources. These are the "original" documents pertaining the question. For me, in ancient history, it might be the writings of Caesar, or the poet Ovid, or Homer, or a papyrus. Primary sources also include inscriptions on stone, pottery evidence, archaeological excavation, etc. The print production of modern academics is called "secondary" scholarship. These are the accumulated theories and studies of academics of the (mostly) modern era. The typical outlets of secondary scholarship, which we utilize in this subreddit, are:
Books on the topic published by academic presses. Not all publishers are considered reliable or reputable sources, and self-publishing is nearly always a red flag for a book that purports to be academic in nature. Most University presses are reliable, as well as a few long-established publishers not directly tied to any particular University. In my field (ancient history), there are many stand-outs. Off the top of my head: Cambridge, Oxford, U. California, U. Chicago, Routledge, Brill, Blackwell, De Gruyter, Harvard, Princeton, etc. In the 21st century, Cambridge and Oxford in particular have a bit of a stranglehold on the academic publishing sphere.
"Articles" (essays, or smaller entries than books) in academic journals. Journals can come out every year, twice a year, four times a year, or whatever else, depending on the journal. They are often referred to by their year and issue number. So for instance, I might refer to an article in the American Journal of Archaeology, year 2020, issue 3, p.391-417, which happens to be David Ojeda's article titled "Fragments of Roman Sculptures from Hadrian's Villa." Journals are typically abbreviated, which can be very confusing for the uninitiated, unfortunately. For instance, the journal above is the AJA. The citation would look like: AJA 2020.3: 391-417. There are thousands of journals in hundreds of various fields, not just in history, not just in humanities, but in every academic discipline. And there are special tools to find what you are looking for in these journals. One such is jstor.org.
Both of these sources feature peer review--that is, when a person submits a manuscript for consideration, it is first either accepted or rejected outright by the editor(s), and if accepted, then disseminated to several other scholars in the field anonymously. They vet it, and then recommend it (or not) for publication.
Typically, in order to access these items, you need privileges at a University library. The local town library will not have such books and will not have access to such journals. In this way, much of academic publishing is gated to University students and above. Anybody can technically buy any published book, but academic books tend to be rather expensive. Journals are even more difficult to access for non-academics. Anybody can search jstor, but most articles won't be available for reading without a login (typically from your home institution).