Is data being sent to/from Jellyfin server encrypted if connected via HTTPS?

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Port 80 is for HTTP traffic. Port 443 is for HTTPS. Yes, connecting via HTTPS will mean your traffic is encrypted. Depending on how much you stream, your ISP will have an idea of what you're doing, but there's nothing illegal about running your own media server.

If you're worried about opening ports, see if you can find one that's open (i.e., not blocked by your ISP) and run a Wireguard node or use Tailscale for remote access rather than contacting them. That said, highly recommend against exposing port 80.