how does your website know DNS server?

Hello. Thanks for your services. I'm migrating some family members. Is it OK to use same account for our house and our parents? Also, I love the iOS app and ability to use HaGeZi.
 

I visited your lovely website and it says: "This device is not using NextDNS. This device is currently using “ATT-INTERNET4” as its DNS resolver." How do you know this? Is there a white paper that explains this and results from test.nextdns.io? I ask since I'm pretty sure my LAN is configured properly to use unbound. I presume this is merely an inference?