Otherwise, it could also be set at the router layer to use the cloud instance to block the ads.
Why? One can set up any device to use the pi-hole machine as their DNS server.
Because iOS apps don't have ads, right?
Apple allows ad blocking software in the app store.
So does Google. In fact nearly every browser in the Play Store blocks ads.
but not chrome, which is the major browser on android...
Firefox for Android has addons, so you can use uBlock Origin. That seems to be the go-to solution for many Android users. But just like content blockers on iOS, it won't affect other apps.