Can security apps scan an iPhone?

No they can't. iOS uses a technique called sandboxing with isolates programs from each other making it impossible to scan other apps.

Security of runtime process in iOS, iPadOS, and visionOS - Apple Support

The built-in Apple anti-malware can scan the entire contents of the iPhone.

Add-on anti-malware apps cannot scan anything in the file system outside their own app-local sandbox, but can scan internet traffic.

Basically, these apps can collect the same sorts of network-related metadata that add-on VPN apps can capture.

There are many vendors that want their apps installed, so that they might collect all that delicious metadata.

Nope, but if you try one, the app will likely come back with some sort of result that tells you that you need to buy more software to fix problems on your phone.....(which do not exist)