They have a sorta proprietary metadata service that is presumably based on imdb, thetvdb, etc. but they also handle detection and collection of metadata regardless of where the information ultimately comes from. It's nothing that Jellyfin doesn't do though.
I'm sticking with Plex since I have the lifetime pass too, but the writing's on the wall, I'm ready to switch to Jellyfin whenever Plex dies or ruins itself
Haven't really looked into it. I have Jellyfin set up and ready to switch over already. Maybe I'll check it out when Plex blows up