They provide a default relay. It’s not clear to me whether you can manually specify a different relay.
https://github.com/n0-computer/iroh/blob/main/iroh/docs/loca...
https://github.com/n0-computer/iroh/blob/main/iroh-relay/src...
If you use iroh as a library, you can specify your own relays.
It is important to mention that relays are interoperable, so you don't have isolated bubbles of nodes using certain relay networks. I can have the n0 relays specified and still talk to another node that is using a different set of relays.