Websites cannot run virus scans.
Virus scans are deeply intrusive activities, requiring access that even native apps for iPhone don’t have and can’t get.
If a website could run a virus scan, they wouldn’t bother with the subterfuge, and with what amounts to a scam advertisement.
They’d just pillage everything directly.
This is probably an advertisement for an add-on VPN apps, and these VPN apps are some oft-unnecessary and problematic apps, and (ironically) a VPN app won’t even remotely solve the problem the advertisement (falsely) claimed you had.
Some background on common scams: