I don’t know the details well enough to explain it, but it’s got something to do with a rapid two-way port guessing game to trick your way through the firewall because both sides are participating. Mafintosh tried to explain it but I couldn’t follow exactly. I just have found that it seems to work every time, it connects really fast, and everything suggests nobody is running any specific server and that I’m not connecting to any in order to make it work. 

The only thing I’ve seen is that some people behind really unusual combo of business networks, firewalls and VPN have reported some issues before, but I’ve not experienced it personally. In fact it’s the only thing that still works even if both of my machines are on separate VPNs. RustDesk, which has a central server, fails to connect when my Linux is behind a VPN.