Trying to understand one of the learning asset

I’m actually learning to create a game all by myself, so the question made pretty silly to ask here.
I’m trying to figure out how this asset works, and now I really want to know how you guys achieved to manipulate different part of the character individually even if there is only one mesh. I do noticed there is one bind pose and skin cluster node used every joint, but I can’t quite understand how it works.
Besides, what ik_foot_root and ik_hand_root do?
If there are some useful docs or books recommended, do tell me, please.
Thanks a lot!