All this is handled by the browser and WebRTC library. It's not directly to do with Construct, so I can't help you I'm afraid. I would guess it either figures out which one performs best and uses that, or alternatively goes through them in order, connecting to the first one that successfully accepts the connection (i.e. only proceeding through the list if a server is down). It might even depend on the browser, and use different strategies on different platforms.