malwarebytes

The free version of Malwarebytes can be useful if you think you may have somehow gotten adware on your computer. Make sure you only download it from the developer's website, not a third-party site. Delete it when you're done.

There's no need to keep it or pay for the premium version.

See also here:

Effective defenses against malware and ot… - Apple Community

Another good tool would be Etrecheck

Download the Application Etrecheck directly from the Developer.

Link below and look for " EtreCheck is identifying a file as malware but the file is not malware "

And this link from same Developer https://etrecheck.com/en/features.html

Excerpt from above

" EtreCheck uses digitial signatures to identify the developer of each app. Files that don't have a digital signature cannot be postively identified. EtreCheck ignores the names of files, which could be fake. It only looks at verifiable digital signatures. A hacker could easily write a virus and simply give it the name "com.adobe.update.plist" or "com.google.chrome.updater".

The optional Power User package allows users to override EtreCheck's malware detection and manually flag a file as safe. "

There is Adware & Malware which will affect the computer and is generally downloaded from “ Shady “ Websites or Developers who Inject this software into the download.

So should this occur ( suggest reading from link proved by @ IdrisSeabright ) to avoid this possible issue .

The Malware / Adware would only effect the User Account from where it was installed.

Additionally, specific to Big Sur macOS 11 and above. The Operating System resides in a Sealed and Read Only Volume that can not be opened by the User