If you want to get serious about it, start by reading scholarly books. Wikipedia is not a bad place to find useful references to other books.

It's very hard to assess reliability unless you already know a lot about a subject, or are willing to wade through reviews by other historians. Your best bet is to choose things that have reputable publishers (university presses, or the "big" non-fiction publishers, like Norton, Knopf, FSG), and to read many things. As you learn more you'll be better at figuring out what seems right and what seems wrong.

If you really want to learn about the primary sources, you look in the footnotes of the books. This is, by the way, why said books are better than documentaries, YouTube videos, and Wikipedia — they cite their primary sources. The things you are talking about are mostly tertiary sources, which is to say, they are reliant on secondary sources. The actual secondary sources themselves (the academic works of scholarship) cite both secondary and primary sources to make their arguments, and so you (and anyone else) could in principle follow up on these sources and check them. The practicality of this will depend on where the source is — sometimes it's a matter of clicking a link, sometimes you need to go to an obscure archive in person to see something (which basically only scholars do).

There's no short-cut here. Primary sources are very hard to interpret and becoming a historian means learning how to do that, over a long period of time. But you can learn quite a lot by reading real historical books on a subject. If you go to a library, or a bookstore, and look at the "World War II" section (any large library or store will have one) you'll find lots of options. Again, when starting out, look for books by professional historians (frequently academics with advanced degrees) as opposed to journalists, look for presses that are reputable, look for books with footnotes/endnotes. None of that guarantees a book will be good or true (scholars argue with each other, as well, and some books by journalists and non-PhDs are good anyway) but it's a place to start.