Hello! As a complete beginner in home networking I am a bit lost with my problem. Maybe someone can help:

Setup: Internet socket in the wall -> Fritzbox-router -> Linksys router with OpenWRT and a VPN (NordVPN)

I have to Networks I can connect to, the Fritzbox-Network (192.168.178.) and the OpenWRT-Network (192.168.1.). Most PCs/Smartphones are connected to the OpenWRT-Network to be a bit more protected with the VPN. Some are connected to the Fritzbox.

Now two questions:

• How can I connect those two networks so that I can e.g. ssh from 192.168.178.10 to 192.168.1.30?
• Who is providing the DNS, when I connect a PiHole to the Fritzbox, set it as DNS-Server and then connect my PC to the other network, which is routing everything to NordVPN? Does NordVPN use its own DNS-Server?

What do I need to learn to understand my own network better?