FFmpeg Unity Bind 2 is open source now.
1. How to use FFmpeg?
Here is a great guide which most people like to start from. If you understand how to use the library which is built for Stand Alone, then it will be easier to work on mobile.
2. What Unity version should I use?
Unity recommend use LTS releases for serious projects. Recommended Unity version to build projects with FFmpeg Bind 2 is 2019.4.11f1 LTS which is covered by QA.