Security Updates

For your peace of mind and well being of the computer suggest downloading a Malwarebytes. It is a Trusted Developer and Well Respected ASC Contributor. The application is free to use or paid for added features. Run the application and if it finds Adware / Malware it will remove it.

As for Virus affect macOS - there are none that are in the wild that self replicate. So do not waste you money on this type of stuff.

As for your initial question - Big Sur runs on a Sealed and Hardened volume that is almost impossible to be altered, modified, or otherwise infected or compromised.

Below directly from Apple

Malware Removal Tool

Should malware make its way onto a Mac, macOS also includes technology to remediate infections. The Malware Removal Tool (MRT) is an engine in macOS that remediates infections based on updates automatically delivered from Apple (as part of automatic updates of system data files and security updates). MRT removes malware upon receiving updated information, and it continues to check for infections on restart and login. MRT doesn’t automatically reboot the Mac.

Automatic security updates

Apple issues the updates for XProtect and MRT automatically based on the latest threat intelligence available. By default, macOS checks for these updates daily. Notarization updates are distributed using CloudKit sync and are much more frequent.

User wrote " So even if something was put on my Mac .. " - have to put this in context if you please. Nothing just gets put on your computer. For something to be installed - it requires the express and purposeful action of the Admin Account User to download and install the object, extension or application. Yes, there are times one may download from a Third Party Untrusted Site and intended to get an application and it is infected with Malware / Adware. So, if this is the reference to malicious code - the Malwarebytes should remove it.

Please detail what possible " malicious code " is being referred to. ?