Is the MacBook Air 2021 protected from viruses?

macOS ships with anti-malware.

Some related reding > Effective defenses against malware and other threats - Apple Community

is the Mac Air 2021 protected from viruses?

Yes, in the sense there are no Mac viruses. Lots of various types of Trojans, but no viruses.

Do I need to purchase a 3rd party virus detection software?

No. A complete waste of your money and system resources.