The advertisement lied about the scan, then lied about the virus, and, well, lied.
The advertisement that lied about that worked well though, because you’re here, asking about an advertisement that lied about the scan and lied about the virus, well, three, or five, or a billion trillion viruses, because the advertisement that lied about the scan and lied about the virus used a word that scares you: VIRUS.
HACKER and DARK WEB are some other popular scare words.
There is malware that exists for iPhone yes, but — if the advertisement that lied to you about scanning and then lied to you about the virus had access to that sort of (expensive!) malware, the advertisement that lied to you about the scan and lied to you about the virus would not have needed to lie about the scan and then lied about the virus.
Further, had the advertisement been able to even conduct the claimed scan, it would have just ripped off everything, as scans are deeply intrusive operations, operations prohibited of even local apps. So even the “scan” claim itself is, well, problematic.
It’s also generally exceedingly difficult to prove a negative; to prove that some complex device is not compromised.
Yes, I’m hammering on the word “lie” there, because lies can be pernicious, and because repeating the lies works.