How to scan my iPhone for virus

Since an iPhone cannot have a true virus there is no way to scan for one. If your iOS version is up-to-date (currently iOS 15.5) and you have not jailbroken your iPhone or side loaded apps you have no risk of malware either.

You don't. There are no known viruses in the wild that can affect an iPhone that has not been jailbroken.

Any antivirus apps you could find in the App Store are basically snake oil.

Why do you think you need to?