I came across what seems to be a great web resource for Audio-Visual Preservation using FFmpeg cross platform.
This resource is released under a CC BY 4.0 (Creative Commons) License and provided “as is” without warranty or support of any kind.

Most of the content is in English, a smaller part in German only
https://avpres.net/

Tutorials (under work)

Especially the latter "FFmpeg Cookbook" seems for me to be a great tutorial and reference using FFmpeg code syntax.

An example:
Filter image  - Pad 4:3 into 16:9
https://avpres.net/FFmpeg/4-3_16-9