Arr and stremio serve different purposes. If you just want to watch content yourself then do stremio.
If you want to keep certain movies yourself and want to supply a streaming service to friends and family then arr is better.
Arr has ability to watch your content with no internet.
If your into foss software arr is also way to go
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