How to secure an unsupported iMac (2015) running Monterey

Although few security systems are absolutely foolproof, most criminals today find it easier to attack you than to go to the work of "hacking" your computer. Thus the floods of scam messages using every form of cheap electronic communication (phone calls, e-mal, text messages, Web notifications, pop-up windows, etc.) known to humanity.

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

When they do go to the trouble of hacking computers – or buying the means to do so from other criminals – they often will target computers belonging to shopping sites, financial institutions, government agencies, etc. – places where a successful securty breach might yield information on millions of potential victims at once.

One thing you can do is to use strong passwords on all of your Internet accounts, and to avoid reusing passwords for different sites.

Effective defenses against malware and other threats - Apple Community applies to Macs (and everything else, for that matter) regardless of their age.

Scroll down to the Appendix for a link to the "and everything else" discussion.

As far as Web browsers go, while Monterey no longer receives the latest versions of Safari, it is enough to run the current versions of just about all major third-party Web browsers. Including Firefox, Opera, Brave, Microsoft Edge, and Google Chrome. The current version of Firefox will even run on versions of macOS as old as Catalina.

So you might want to install Firefox alongside Safari.