What exactly do you mean with “while map load process and character creation”?
I think a good start is AGameMode as many things are handled there: AGameMode | Unreal Engine Documentation
But I agree with you that it is pretty hard to find kind of a starting point there, it’s probably the best to look for specific things, at least that is what helped me much, to look for “What exactly do I want to do right now” and search for how to achieve that. Slowly and bit by bit, but as soon as you have a good understanding of how Unreal handles things it’s not that hard anymore.