Basically yes, you find which values in memory are changing when you do actions ingame, and you can also figure out which functions run etc. Then you can for example replace the function call with your own function. I can’t do this myself and it is very time consuming, you can watch tutorials on IDA or Ghidra to get a gist.

To be more exact, usually games additional dlls, and they will first try to load ones from the same folder as the exe, so you can modify or add a dll and whrn the game calls it make the modifications you need. So no need for a secondary process that monitors the game, but it is kind of hacking the game.

But often it is not that hard, c# or java dont get compiled into machine code and usually keep more metadata about the code, sometimes basically the original source code. Unity, Godot and UE all have some level of support (or reverse engined knowledge) for modding even if the game iteelf doesnt explicitly care about it. Of course it is much easier is the game is made with modding in mind though.