As a starting point perhaps you can start from warwound B4A Open file from Uri solution to found some inspiration for some bittorrent torrent files in it's DirInternal and a second app a bittorrent client that needs to open one or more of the other app's torrent files?
It's rather hard and complex subject. And question is about a lib to built-in the torrent files transmission between app instances.
So, it's interesting a full solution.
It seems, never released for B4X yet