STUN gives back your public IP:port, TURN gives you assigned proxied IP:port.

You take that data and send to the peer over signaling connection, and they call you back on that IP:port. Most NAT implementations make and keep a temporary mapping between public port to private IP consistent[1] for few minutes, and not completely random per destination[2], so it usually works.

1: e.g. router.public.ip.example:23456 <-> 192.168.0.12:12345

2: e.g. if stun.l.google.com:12345 sent from port 23456 but if yourfriend.router.ip.example:12345 sent from port 45678