There’s a series of novels kinda based around that idea. Magic 2.0 is the name of the series. Basically various hackers from different time periods discover a file that controls the whole universe and learn how to edit it with code/macros and become wizards because of it
Jellyfin doesn’t need any particular setup to work directly from LAN because it doesn’t ever try to use a central login provider the way Plex does.
The only reason OP is struggling with it is because they set it up so that they can only connect to it via Tailscale.