They are both media servers. Plex is closed source and requires you to make an account with them to use whereas jellyfin is open source and requires no account.
Basically when you set them up you just pick a directory for movies and tv shows and any video files in that directory can be streamed thru the app or web ui
I prefer jellyfin because of the differences i mentioned earlier but i use both (you can point them at the same directories). I haven't really had any issues with jellyfin performance but run plex as a backup.
Plex has plex shares too where you can stream other peoples content from your server. Idk if jellyfin has that feature
Bodhi Linux. I have an old hp desktop and it only has 2 gb of ram and 2 cores. I wanted a real lightweight distro and settled on arch linux but one day i tripped and knocked the tower over. When i booted it back up i had the infamous blinking underscore. I tried reinstalling arch multiple times and it kept failing, so i tried a lightweight arch based distro called archbang, same issue. I tried manjaro same issue.
At this point i wanted to try something not arch based but wanted something that came with minimal preinstalled programs like arch. Research led me to bodhi which is a light weight distro based on ubuntu. Installed with no issues and been using it ever since, about 3 years now.
Don't know what i damaged on the motherboard but it must have been something integral to arch based distros, but i'm kind of glad it happened because i love bodhi now
It has allowed an ancient computer to do so much. I've run matrix servers and web servers, written my own webapps to run on it and so much more