It's using a turn server to provide the DERP feature (in the case of the quick setup it's coturn but you can use other turn servers). So to create this you just set up more turn servers and add them. It's fairly simple.

I couldn't find a specialised page for it. But here they mention that the relay service can be self hosted: https://docs.netbird.io/about-netbird/how-netbird-works#rela...