Blender
https://www.blender.org/
Blender 2.9 Retopology Getting Started
https://www.youtube.com/watch?v=e47RJuxs6ro
Retopology for Beginners in Blender 2.8 - Retopo the Correct Way
https://www.youtube.com/watch?v=CuQzPDs99yM
Polybuild Quick Tip:
https://www.youtube.com/watch?v=oO6o1O6yV6E
Blender Addons:
3D Print toolbox
Loop Tools
XNormal Tutorials:
https://xnormal.net/
https://www.youtube.com/watch?v=5n8XsbrUxNI
The fundamentals of perfect baking
https://www.youtube.com/watch?v=kGszEIT4Kww
Inverting a normal map in blender
https://victorkarp.wordpress.com/2020/06/26/inverting-a-normal-map-in-blender/