It may not use it if you don't explicitly need it, there could be out-of-band detection that adds a TURN server or some other OOB relay if it's needed. I've also seen comments online that say Chrome in particular can support TCP for WebRTC which would negate the need for a relay (as normally only DTLS over UDP is used). But based on my understanding of how WebRTC and NAT works, using the typical UDP approach for bidirectional communication over symmetric or CGNAT absolutely should not work, barring some other method of NAT traversal such as a browser-based UPnP client or such.