The domain side of things would be entirely done at your registrar (the company you paid for the domain name).
Firstly, you’ll need to make sure that your internet connection (where your Jellyfin server resides) has a static IP address. Many residential-grade connections don’t have a static IP address, and if this is the case for your connection, you’ll need to go a step further and arrange a “dynamic DNS” service.
If you have a static IP address, simply go to your domain registrar’s DNS manager and add an “A” record for the host address. For example, if you want “jf.mydomain.com” to point to your Jellyfin server and your home IP address is “188.8.131.52” you would add an “A” record for “jf” with a value of “184.108.40.206”
If you have a dynamic IP address, you’ll need to use a service like DynDNS, ZoneEdit or any of a number of others to maintain a mapping to your ever-changing IP address, so remote users can find your server. Your domain registrar might already provide such a service, but if not, there are plenty of services available - just do a web search for “Dynamic DNS.”