You type the ip of the rpi on the router, so from an external call the router will forward it to the rpi. Or I don’t know what is your question.

Things may seem automagical in the networking scene, but you can config anything the way you want. Even in nginxproxymanager you can edit the underlying actual nginx configs with their full power. The automagic is just the default setting.

External 80 to internal 80 and external 443 to internal 443

With this config you don’t have to deal with ports later, as http is 80, https is 443 by default.

If you run some container on port 81, you have to deal with that in the reverse proxy, not in the router. E.g. redirect something.domian.tld to 192.168.0.103:81

If you use docker check out nginxproxymanager, it has a very beginner friendly admin webui. You shouldn’t forward the admin ui’s port, you need to access it only from your lan.

Install docker, grab the official docker compose file, then docker compose up -d.

Details: Look up how to install docker on elementary (I guess it’s sudo apt install docker), than you don’t have to care about the distro after that, docker works the same way everywhere. You can find countless tutorials on this, and they should work

Are you trying to install jellyfin server or client?

Elementary 8 circe is based on Ubuntu 24.04 LTS, which is based in debian 13, which has a codename “trixie”.