Virus protection for my iMac (21.5-inch Mid 2011)

Pegasus is only used against iOS and Android mobile devices, so you're protected against this specific spyware attack. To keep your Mac secure from other attacks, make sure to take standard security procedures.

Set up your Mac to be secure - Apple Support

• Keep your Mac up-to-date with the latest security patches. The latest version available for iMac models introduced in 2011 is macOS High Sierra 10.13.6. You can update your Mac using the App Store on macOS High Sierra. It's also important to keep the apps you use, especially web browsers, up to date. The latest version of Safari for macOS High Sierra is Safari 13.1.2. You can also use browsers that are actively maintained for macOS High Sierra, like Google Chrome. The latest version of Chrome is Chrome 93.

Update macOS on Mac - Apple Support

How to manually update apps on your Apple device - Apple Support

• Make sure to use secure passwords to sign into your Mac. Don't use passwords that you use elsewhere or passwords that can be easily guessed.

Learn how passwords are used on Mac - Apple Support

Tips for creating secure passwords on Mac - Apple Support

• Encrypt the data on your Mac with FileVault. FileVault keeps your files safe so they can't be read or modified without a login password.

Encrypt Mac data with FileVault - Apple Support

How does FileVault encryption work on a Mac? - Apple Support

along with the good advice you've already received, i'll recommend you read Effective defenses against malware and other threats - Apple Community. it's a long article, but it's worth your time to read every word.