If you use the app then you have to pay for their subscription to use DNS features. If you don’t pay then you’re really just limited to adblocking in Safari. The profile I linked is free and will work for all apps. It’s not as robust or customizable as the AdGuard app, sure, but I’ve never had any real issues or need for anything more. Plus it’s one less app to have installed.

It’s personal preference mostly.

You can use AdGuard’s DNS to block ads system-wide. You don’t need to install any app.

https://adguard-dns.io/en/public-dns.html

Just scroll down to method 2 and download the profile from the iOS section. You’ll need to be using Safari for the download to work properly. Then if you ever need to disable it the setting is in Settings > General > VPN & Device Management