You just send packets to the other party's address and they send packets back to yours. Both parties know their address and you don't need a relay in the middle.
replyIt's not really relevant in this case since one endpoint is a massive server farm.
replyIt is because most of their complexity is in routing packets. With IPv6 you can just have the thing handling the conversation directly addressable by the client. The last 64 bits of a v6 let you have billions of instances in a region.
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