I have not found any site that breaks things down for you, here and there you might find mention of class X Y or Z that is fun/useful, tutorials here and there (which MOST of them alllll cover the same stuff, and then most of those are pretty terrible, especially the majority of the video ones)
A lot of the forge stuff is documented/commented, but from what I've seen a vast majority of the vanilla code is not.
Some of it is easy to understand, some of it feels like a spaghetti mess rabbit hole.
Best things to do:
- dig in and just read the code for stuff you're interested in
- google, google, google! And forums search on here
- Post questions when you can't find what you are looking for. This is the trickiest one probably, because you have to pose your question correctly, and sometimes you don't know you're asking the wrong question. Use as much detail as possible to get the best answer you can.
I'm not really making a mod, but I love to tinker, so I spend a lot of time messing with concepts as opposed to finish products, which sounds similar to your original question. Bad news is, you can't be lazy on this one, you have to actively mess with stuff/read code/etc, because most people on here are doing their own things, and while we enjoy helping, most appreciate when people at least try to find their own answers