I have a RasPi 3B+ and some female header connectors. On the V1, I can see that the RasPi plugs its GPIO headers directly into the Nyboard. I assume this is how it's powered but I'm not sure.
Yes, it is. You can find the manual for NyBoard V1 here: https://docs.petoi.com/manual-for-nyboard/nyboard-v1_0
There's a independent 5V-3A DC power for Pi. You can directly mount Pi on the Nyboard since you have solder the 5*2 pins.