So for some reason Apple keeps using their DNS resolver even with a custom DoH resolver configured, but in my testing it didn’t affect the blocking capabilities of NextDNS at all, meaning that the answers from their resolver are just ignored (or used for some other purpose). The way NextDNS knows that you’re using another resolver is by letting the browser resolve some unique hostnames, so that way it will show up even if the answers from that resolver aren’t used. As to why Apple does this I don’t know. In theory it could be the case that Apple just used whichever answer arrives first and that NextDNS just happened to be faster in my testing, but that doesn’t match with how it’s documented in their PDF.
Which one to pick (if you don’t just want to use them at the same time) depends on what your goal is. I use iCloud Private Relay + NextDNS + AdGuard, but nowadays I mainly use another browser with a built-in adblocker, so iCloud Private Relay and AdGuard aren’t used in that case.
I use NextDNS everywhere I can and use a list that prioritizes not breaking anything. It’s a nice backstop. It’s not a replacement for an in-browser adblocker in my opinion, unless you don’t care that it’s less effective.