I dont think you can selfhost media server on xbox, but both jellyfin and plex are amazing. If you want to run media server you should have x86 or arm based PC. Old laptop or desktop PC or SBC like raspberry pi is great starting hardware if you have one laying around. If you want to buy hardware lot of people recommend used SFFPC like intel nuc or lenovo thin client. You can run both plex and jellyfin server at the same time, they are quite lightweight and easy to install. If you want to share media with a friend plex is better option afaik, but I dont need that and like jellyfin more.
For a player, if there is no jellyfin for xbox, just use plex. Android tv supports both