Can Jellyfin be accessed outside the network?

Yes. Jellyfin is just a Docker container. It can be accessed the same as any other Docker container. Plex simplifies this because they can basically configure all the DNS stuff for you, because it goes through their DNS. But either can be accessed.

Or it requires a lot more work (and maybe some paid service) to do.

More specialized knowledge to configure Tailscale or a reverse proxy, yes.