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The server is used to exchange connection information. The A/V call itsef is peer to peer (or relayed via TURN server if they cannot establish a direct connection).

In order to establish a peer to peer video call you need a signaling channel that both peers know about.

> Calls are end-to-end encrypted and use a direct connection between the peers whenever possible, but fallbacks are also in place.

I imagine this means the 2+ clients use XMPP to negotiate direct connections to one another that are better suited for audio/video streaming.