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

The docker implementation on synology is pretty bad. I ran my setup on there for about a year before I got fed up. Ended up picking up a mini-pc and installing unraid on it, which has a much better way to run and manage docker containers. Only downside is having two machines now with the storage pool over the network. A bit more complications rather than direct attached storage.