Oh wow, I have done some work with Netcode, but haven't connected any games to anything officially yet. What is a relay server? I have some googling to do it seems.
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Oh its nothing but a way to make p2p networking easier for clients by opening a secure port for clients to join(this means clients don't need to do router configuration, firewall setup, etc.)
Here's Unity's docs for it if you wanna know more about it: Unity Relay