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/