1. Only B4A and B4J can share the same libraries. The libraries that can be shared by B4J are listed here:
https://www.b4x.com/b4j/documentation.html
Most of the libraries cannot be shared as they are based on Android APIs or APIs not available in Android.
When I create a library that can be shared (like MQTT client) then I post it in both B4A forum and B4J forum.
2. This issue was discussed many times before including a failed attempt in the wiki:
https://www.b4x.com/android/wiki/index.php/Libraries
3. There is already a list of libraries per product with a very good search engine and it is always 100% up to date.