While its trivial to edit DNS settings for wifi, its actually quite difficult to change your DNS server on the cellular profile on iOS as comment from Easton here rightly points out. I was kinda surprised the first time I found out you can't edit the cellular DNS server settings via the phone's Settings app.

One option that works for me to get custom DNS on iOS cellular connections (I like PiHole ad blocking on my phone) was to setup my own VPN connection to a VPS instance running PiHole for DNS and WireGuard for the VPN. Lets me get custom DNS, pihole adblocking over cellular so long as VPN isn't blocked by your cellular provider etc. Was two trivial Docker containers to get running, costs very little in AWS.

Same trick also lets me access region blocked TV services from my iOS devices over US cellular simply by turning a VPN on - I just stand up the containers on a VPS host based in source country and connect to that.

Yes, but you can't set custom DNS for cellular networks without a configuration profile or an app, so it's unlikely that most people have that set.

Depends on the ISP. If they block or re-write DNS packets, then setting your own servers wouldn't fix it. That's a real thing people see in the wild: https://superuser.com/questions/897543/how-can-i-check-if-my...