How to identify real and fake security warnings in Safari on iPhone

It is an advertisement.

Subtle clues that it’s sketchy:

Trademark: Apple uses “macOS”, and not “MacOS”.

Phrasing: “until you have taken all the suggested steps by authorized Antivirus”. Apple isn’t known for poor grammar. Depending on where you are located, the spelling “authorised” can be another clue.

“Under threat” and “more damage”? Which is it?

If macOS finds an issue, it’ll either fix it, or will report it directly, and not via some Safari popup. You might get a link to an Apple Support article (at the apple.com website) shown, too.

Websites cannot scan clients. Scans are deeply intrusive. If websites could scan, they’d just upload everything and skip the subterfuge. So finding malware is quesrionable.

I’d look to get an ad blocker installed and running (Wipr is one I’ve had some success with), and to disable Safari pop-ups if those happen to be enabled.

I’d also be generally more mindful of anything associated with Facebook and any other social media, including things the social media service profits from (ads, sales commissions, etc), and (more subtly) things the social media service doesn’t profit from and doesn’t boost.

Fake.

Aside from the obvious clues, some of which MrHoffman identified, how can you identify them on your own? Easy. They're all fake.

There exist no such legitimate warnings. If you find one, it's a fake. Simple.

Read Phony "tech support" / "ransomware" popups and web pages - Apple Community for a few entertaining examples.

That is a scam attempt. They are trying to get you to purchase unnecessary software and give them your money or give them your personal information. Close out of the tab and don’t interact with the message.