does my imac have built in virus protection?

I have never uses any 3rd party virus or security product. See: macOS - Security - Apple

The short answer is yes. I fully support the other answers here. Never install any antivirus or Mac cleaner apps. They will only do harm. Just so you know, the current Apple OS, Catalina, is further protected. All of Apple's apps, etc. are installed in a portion of the HD that is write protected. It can not be changed or messed with. The things we install are placed in what is called the HD Data. We are very well protected.

Read: Effective defenses against malware and other threats - Apple Community, by John Galt.