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It's using WebRTC's P2P. It does require internet but the middleman is absent in file-sharing.

WebRTC's P2P does require servers; and sometimes even a middleman (TURN). OP wrote about it in https://dikshantraj2001.medium.com/nat-stun-turn-and-ice-466...

Glad you liked it :)

If at least one party is behind NAT, it's a dead end. I wouldn't trust anyone who makes a false claim it's possible.

Please Google udp hole punching

In order to do hole punching you need a centralised STUN server.

We need a STUN server to traverse NAT and get IP addresses but it does not use server to store or share the file. Only ice candidates are gathered, discovered and a handshake is facilitated by a STUN server. If that is not possible it shows error.