How would I go about editing / extending functionality in base C++ classes (for example parent character class) then compiling that to use ingame for anything inheriting it? so far all documentation I stumbled upon only explains how to create new ones that use the default character C++ class as base, but how do I actually modify and compile/use the default class itself?
Short answer: You don’t/shouldn’t.
Long answer: You can download the engine source from github and compile the entire engine. Then you can make modifications to the default and have them affect the game. But even then, you really shouldn’t. You should really only modify the engine to address bugs or address some other thing that is lacking that can’t be fixed any other way. You shouldn’t make any gameplay specific changes through engine code.
You could split the difference and copy the default character source code to make MyDefaultCharacter, but then you’ll loose out on any fixes from Epic that are done to the original character class.