Do I need a security app for my iPhone 16?

You're getting advertisements, and specifically sketchy advertisements, these sketchy advertisements for sketchy vendors offering sketchy apps for reasons of profits, and these sketchy apps that will quite possibly sketchily collect and track you for sketchy reasons, because sketchy ads and sketchy vendors are sketchy, and all this sketchiness for little or no benefit to you.

The sketchy advertisers and the sketchy app vendors can certainly profit from their sketchy ads, and words including "HACKER" and "VIRUS! VIRUS! VIRUS!" have worked well for the sketchy advertisers selling sketchy apps from sketchy vendors.

The shorter version of that is "no, you don't need add-on security apps."

PS: While it is possible for an iPhone to be hacked, that's involved immensely-valuable exploit tooling (and the exploit tooling includes what amounts to its own built-in jailbreak), and exploits using that tooling have been vanishingly rare and very much targeted at specific people, based on available information.

No app is needed. A non-jailbroken phone cannot be hacked. Ignore any suggestion to get one.

It is a scam. You do not need one. I'm guessing you are seeing this while on a website? If that is the case, it is the website allowing that through.