Rigging is a super complicated thing.
I don't have any notes on how they are set up because it was custom built to solve the particular problems the character design threw at me.
I can give you a few words of advice when reverse engineering (which is how I learnt).
Just focus on one area at a time.
Try to make it as simple as possible but no simpler.
And try a bunch of different things with constraints in different orders.
And stay consistent with your naming convention.
It's a matter of learning small parts at a time and then putting them together.
Good luck man, rigging can melt your brain but it's very rewarding.
