How do I do a virus scan on iPhone 12?

There is no virus scan that you or anyone else can do on your phone. You may have been scammed by a website claiming to have done a scan of your phone and telling you that you have a virus, but that is just an attempt to get money out of you to fix a problem you do not have.

Do not attempt to purchase a Cleaner or Virus Scanner for your phone? If you are having a problem with the phone, describe the issue in detail.

iOS / iPadOS devices cannot be infected** with Viruses / Malware / Spyware unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed them on your device or/and have Jailbroken. 

**The primary reason for this is Sandboxing. All third-party apps are “sandboxed”, so they are restricted from accessing files stored by other apps or from making changes to the device. Sandboxing is designed to prevent apps from gathering or modifying information stored by other apps.

Security of runtime process in iOS and iPadOS - Apple Support

The sandbox on an iPhone is a security feature that creates a restricted environment for each app to run in isolation from other apps and the operating system. It is a core component of iOS's security architecture and plays a crucial role in making iPhones more secure.