What should I do if my iMac shows virus alerts?

See the information in the following article for disabling annoying browser notifications that generally are peddling malware & scams:

Stop unwanted Notifications - Apple Community

Also see:

U.S. Federal Trade Commission – How To Spot, Avoid, and Report Tech Support Scams

"Tech support scammers may try to trick you with a pop-up window that appears on your computer screen. It might look like an error message from your operating system or antivirus software, and it might use logos from trusted companies or websites. The message in the window warns you about a security issue on your computer and tells you to call a phone number to get help.

…

If you get this kind of pop-up window on your computer, don’t call the number. Real security warnings and messages will never ask you to call a phone number."

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

100% scam and fraud.

There are many sites out there that masquerade as real sites, just waiting for some unsuspecting victim to typo a web site - just like you did. They rely on the hope that you don't realize you typoed the site, and that you trust where you think you are.

in 100% of these cases, the site tries to trick you into thinking your machine, your data, or your money is at risk and that you need to call/click/do something now to rectify the problem. In this case they tried to trick you into thinking your machine was infected with a virus.

IF you had followed their instructions you'd have ended up paying some scammer for 'virus removal, or even inadvertently giving them access to your computer (under the guise of enabling them to 'fix it' remotely), at which point they either download all your data, or hold you ransom in some way.

The good news is that if you didn't click any link they offered, and didn't call any number they proposed, there's absolutely nothing they can do to affect your machine. It's all about tricking the user (you) into doing something under stress.

If you did click their link or call their number, then take steps to secure your system now.