Selfhosted

Lots of questions, let's take it one step at a time. You have a domain, now you can point it to your public IP, so that whenever someone tries to access example.com they ask their DNS server and it replies with 10.172.172.172 (which btw is not a valid public IP). Now that request will hit your router, you need to configure your router to redirect ports 80 and 443 to 192.168.200.101, that way the request to example.com gets to your local machine.

Ok, so now you need your local machine to reply on that port, I recommend using Caddy it's very easy to setup, but NGIX is the more traditional approach. A simple Caddy config would look like:

example.com {
    respond "Hello"
}


jellyfin.example.com {
    handle {
        reverse_proxy http://192.168.200.101:1020/
    }
}

So after the request reaches Caddy it will see that the person tried to access, example.com and respond with a "Hello".

If instead you had tried jellyfin.example.com the DNS would have sent you to 10.172.172.172, your router would send that to 192.168.200.101, Caddy would then send it to 192.168.200.101:1020, which is Jellyfin so that would get returned.

There are some improvements that can be made, for example if both caddy and Jellyfin are docker you can share a network between them so Jellyfin is only exposed through caddy. Another possibly good idea is to add some authentication service like Authelia or Authentik to harden stuff a little bit. Also as you might have noticed Caddy can forward stuff to other computers, so you can have one machine on your network exposing multiple services on multiple machines.