I'm gonna be blunt honest: if wish I had to to explain step by step every single aspect of game dev to any of you guys... but that's just a sweet dream. There is a lot to learn and understand, and there are plenty of resources already on youtube to help you with that.
But I can point you in the right direction.
There is pretty much nothing to explain (for a CUA) than simply learning how to do a rigged character in Unity. You would need to understand rigging, bones and all that stuff.
So, first, simply start
from Mixamo's system. It will give you all the basics of importing a rigged and animated FBX.
From there, you will have a proper avatar that has a state machine with a (or a handful) of animations attached to it. Then, to control the animations, you will need to understand the basic of c#, working with components and changing animation states.
This is where community's content comes in handy: you can look in my code how I did it. You have a pretty solid base to work with and understanding how you can control a rig through a VaM script.
This is not magic tho. You will not be able to use my plugin on your characters just because I'm working with a really specific setup... you will need to code your own plugin
I hope it helps at least a bit.
Start simple ( a single character, a single animation or two ), then... when you grasp all the concepts behind that, expand your system.
