If you are wondering (as I did) how they can know what is your DNS resolver, they simply makes the webpage load some JS from a random host, like "https://853af2kklyt-dda385.test.nextdns.io/". Of course, as this host cannot be cached anywhere, their DNS are hit by your DNS resolver, thus they can know the IP of your DNS resolver. In my case as I have a DNS server at home, it displayed the name of the AS of my provider.