How do hackers exploit advertising apps to hack iOS devices?

They don't. Hacking the OS itself is nearly impossible.

The main threat is social engineering. Which means you get a false text, email, or web page/popup that claims you've:

1. Had a charge placed on your account, and oh, call this scam number to have it reversed.
2. Your device has xxx number of viruses, and oh, call this scam number to give some crook direct access to your device.
3. Anything you've seen that's similar.

None of these are hacks as hacks involve directly altering a device of the OS running on it. What you're referring to are all lies to fool the recipient into doing something unwise.

Slyshacked wrote:

How do hackers use advertising app to hack ios

I assume you're talking about this story: ‘Nightmare Scenario’: Zero-Click Spyware Is Infecting Phones via Targeted Ads

If you hit the paywall, here's an Apple News link: https://apple.news/A8eOWvuPaTQukunHBEjFXOg

This is not something that end-users need to worry about. It's no secret that state-sponsored hackers are able to hack consumer operating systems. People who do this kind of work for the government are professionals who are very good at what they do. They're paid by your tax dollars.