Hi! My problem is that I don't know how to add custom structures, and I couldn't find any good tutorial that's up to date to 1.15 or 1.16. I know you have to do stuff with Features, but I don't really know where to start. If anyone could link me to an up to date tutorial or could explain what I need to do that would be great. Thanks!

Unlikely that there will be a tutorial for this, at least one that works well. But one of the greatest tutorials out there is just reading the Minecraft source. So, go do that and take a stab. You'll probably have to decide whether your structure is stored in an nbt file or not as well. You might want to try reading through and creating your own implementation from there. It's probably good that there are no tutorials for somethings at times since it will show how well you can apply your knowledge of Java in the current situation.

I'm sorry, but I've been sitting here for 2 hours now, squeezing my eyes together and looking at Minecraft's source code, but I generally don't know what they are doing. This is mostly due to the fact that the code is obfuscated, and there is generally not really a way to know what 'func_230364_a_' means, or 'int p_230364_3_'. One of the ways to do this, is to have some context around it, and seeing what the other methods around that one do. But they are obfuscated too, leading me only into darkness. Now my question is, is there some deobfuscated form of Minecraft's source code at wich I can look at, and know what they are doing? Thanks.

Read this if you want some reference. It'll tell you where the source maps are located. If you want to know why Forge doesn't use them, read this.