This lets you connect to and play another friend's Jellyfin rather nicely. I think it's also so the HTML and JS that forms the web interface can basically be the app as well.
> And it doesn't seem to want to accept any answer as to what the server URL is.
Should be "http://192.168.100.xxx:8096" - make sure your jellyfin isn't just set to listen on localhost only though.
They've sort of conflated three different ideas - one, the non-browser clients are separate and need a way to select multiple servers from a common wrapper app; two, for development purposes separate the server of the UI package from the backend being accessed; three, have a server based application.
None of these items are uncommon, they are just commonly solved by making the separate front-end server an exception rather than a weird and senseless default. Most people solve this by making the dev mode case an exception.
Moreover, why not just add a button to the UI to automatically connect to the _UI serving server_ instead of having the user type it?
I love jellyfin, but this stuff is just terrible design for out of box experience or connecting from a new client instance, and it breaks when you're traversing a network boundary with something like Tailscale.