How the enemy character system works,if this it isn't secret?
Technically, no. But I did follow some tutorials to learn how some things work, like how the "Spawn AI From Class" node works. but they were multiple ones and I learned certain things from posting discussions about how this works or how to do this to make this happen sort of thing. What I recommend you do, is just work on your games, and once you encounter a problem, look up a video or post a discussion on the unreal engine forums.