techsupport

I have two routers. One of them encrypts everything with a wireguard interface and routes all traffic through my VPN provider. The other one is "plain", unencrypted, and it is used for services that need a public IP. If all these services need is a public IP address that leads to the router's wan port, I can still encrypt my DNS traffic, I thought, so I set up DNS over https (DoH), using my VPN provider's servers. To do this on OpenWRT, which is my router's operating system, I needed to download any one package that provides this functionality. I couldn't just configure ports, firewall rules, NAT or whatever and be happy... Of the settings that this DoH package came with lets you choose a "bootstrap DNS". It was configured to Google's and Cloudflare's servers by default, but I changed it to Quad9's. Everything works fine. My VPN provider confirms that I am using their DNS servers and that my DNS traffic is encrypted.

What is going on under the hood? What is this "bootstrap DNS"? Why isn't it as easy as exchanging public keys, as with regular traffic that goes through my VPN router's wireguard port?