Beware though - NextDNS is good but their servers being hosted in France the geolocation gets you IPs in france (when accessing Google properties for example) which slows things down a lot - 30-50ms vs 130+ in my case.
They should have thought about this - it is possible to force resolve the individual domains in the NextDNS UI but that gets problematic fast.
Did you create your personal configuration? When you do, their server should be detecting the geolocation of your IP and assign the nearest ones (what you see at "DNS Servers") to that configuration like how CDN works. You can also check the assigned region at https://ipv4.dns.nextdns.io/ping.
If you want it to just always use what you've configured and not have to follow the changes they make you can set network.trr.mode to 5 in about:config. 5 signifies "off by choice"
If you just want to know how to toggle it normally in case something goes weird with one of the changes it's in the network settings part of the settings page.
They should have thought about this - it is possible to force resolve the individual domains in the NextDNS UI but that gets problematic fast.