Heavy construction on this page. Got anything you think should be here please poke dinusty.
The mouth
ArtStation - So.. How to mouthbag/room for teeth?
Mouthbags (room for teeth) for characters
www.artstation.com
Hair cards
ArtStation - So.. How to haircards?
How i approach haircards and different softwares
www.artstation.com
Skin shader for Unreal 5
ArtStation - Skin Shading in Unreal Engine 5
www.artstation.com
For polygon budgets see the characters section in 
3D 3.6 Polygon Budgets