I've tried to listen to some podcasts on YouTube music. It really doesn't work well compared to Pocket Casts.
They need to separate the music and podcasts more clearly and make managing podcast playlists more intuitive
My favorite part of YT Music is how it's a terrible interface for YT videos. Second favorite part is the total mixing of music playlists and video playlists.
My understanding is it is a file linked to your account that links to the epub file. It lets you and only you download your copy of the epub file you purchased.