Can someone briefly explain how me and my friend can connect to each other without a central server in between? Don’t you usually need port forwarding rules on your modem for that? How does ZeroTier accomplish this without the need for that and central servers?
There are central servers involved, the root servers know the address information for your two clients, as provided by the clients themselves. When the clients try to contact each other, they are handed this information by the root servers and proceed to contact each other directly.
To get around the NAT problem, hole-punching is used to allow them to contact each other.
My terminology in the above might not be correct, just that’s the general process.
The beginning of the manual explains in more detail.