imac: File privileges for ‘staff’ and ‘everyone’ were changed to ‘read and write’ instead of default ‘read only’ I did not initiate this change.

Personally if it was my system, I would make sure to manually transfer my data to an external drive. Then I would perform a clean install of macOS by erasing the hidden Container. Then I would either restore from a backup made before the incident occurred, or I would manually transfer my files back and manually download & install my third party apps. This is the only way to be sure your system is clean & stable and it will probably take less time than trying to figure out if something is wrong and how to fix it.

Definitely agree that a Mac does not require any anti-virus apps, cleaning apps, or third party security software since they usually cause more problems than they solve since they interfere with the normal operation of macOS. macOS already provides great built-in protection when also paired with following safe computing habits as outlined in this article written by a well respected forum contributor:

Effective defenses against malware and other threats - Apple Community

w0t3n wrote:

I discovered file privileges were changed from ‘read only’ to ‘read and write’ for all users,

If true, this is not normal and warrants further research; however there is not enough information in your post to figure out how or why it occurred. You imply that it affects all files for all users. Are you certain it's *ALL* files and not just a certain folder or group of files? Have you checked multiple folders and their contents?

That said, your experience of being redirected to a malicious website when installing McAfee is very suspicious. Are you certain you had a legitimate copy of McAfee in the first place? And was the copy you actually installed with help from McAfee phone support from the same disk or download you used the first time when it got redirected? Are you certain the phone number on the physical box was an actual McAfee tech support line?

Despite your experience and your obvious interest in protecting your Mac, most advanced users in these forums would advise against installing any virus app, including McAfee, as unnecessary and potentially problematic in their own right.

Well, you said: "when I was installing an antivirus (mcafee) it redirected to a malicious website," so the redirect certainly appears to have happened while you were installing McAfee antivirus and we could only conclude that it did have something to do with your antivirus software installation.

No legitimate software would redirect itself to a malicious website; and if it did, perhaps you have browser redirect malware. It would be advisable to check ... Malwarebytes is one product you could use to check; the trial version is fully functional for 14 days.

"the issue is due to flash drive file transfer"

That statement confirms to me, again, using flash drives for backing up is a bad idea.