You can replace the default web view, although I believe it requires root access. I've been using https://www.bromite.org/system_web_view for a while and it works pretty well.
So the only way you can actually use your ad blocked system wide is through a series of hacks
https://github.com/bromite/bromite/blob/master/FAQ.md
That’s definitely a lot easier than just…going to the App Store.
Of course when your phone is using an OS that is created by an ad tech company, they aren’t going to make it easy to block ads system wide.
Its why it's important for folks like yourself who are pro-Apple to push for Apple to open up. That way we can have the best of both worlds (if you believe the marketing, like it would appear you do). I'll use the most open device at the end of the day.
You don’t just get a pop up that allows you to enable it. You have to go into settings and get a scary warning.