Traffic goes through their DERP servers if all p2p connections techniques fail. For example Tailscale tries to do UDP hole punching to create direct connections

Traffic goes through their DERP servers if all p2p connections techniques fail. For example Tailscale tries to do UDP hole punching to create direct connections between nodes even if there isn't port forwarding. One of the main reasons behind Tailscale is to do just this, connect nodes directly rather than pushing data through some central hub.