How can I block ads from sites on iPhone?

There are two ways app developers make money.

1. Purchasing their app

2. Adverising revenue.

There really is no cure all app for removing advertisements. Purchasing their app will remove the ads.

If you asking about removing ads on websites, there are many out there to try, I would suggest trying a few until you find one you like.

Two very simple and often effective Safari solutions to avoid many web annoyances is to learn how to use both Reader (Hide distractions when reading articles in Safari on iPhone - Apple Support, and Distraction Control (Use Distraction Control in Safari to hide items on a webpage - Apple Support

If you want more aggressing content filtering, using a filtering DNS provider such as NextDNS is a very effective method for blocking ads and annoyances from the web while also providing security and privacy benefits. The benefit of using DNS to do this instead of a local blocker is performance. Filtering at the DNS provider is performed before the unwanted web content even reaches your phone. I would recommend testing out a service such as NextDNS (https://my.nextdns.io). Follow their instructions to create an account and to download the app for iPhone. The app is useful in that it allows easily disabling the use of the service should you have a connectivity issue. Their default filtering includes blocking the sources of known malicious content as well as basic advertising and tracker blocking.

Please do understand that using an alternate DNS provider (or a VPN) involves significant trust in the service provider that you choose. Therefore, please do some research on the topic and any choice of DNS provider that you choose. Some respected filtering DNS providers to consider are names like NextDNS, AdGuard Public DNS, Mullvad, or Quad9.