Phishing popup on iPhone

• Quit Safari: Quit and reopen an app on iPhone - Apple Support
• Clear saved data related to that website (pr clear all): Clear the history, cache, and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support
• Restart Safari, and check again.

Dismiss and ignore it. It's a scam.

(256111)

Everything it alleges is a lie. Phony "tech support" / "ransomware" popups and web page - Apple Community has a few examples and a description of how to get rid of them, illustrating what MrHoffman wrote.

Another typical example, which appeared as an advertisement hosted by an otherwise legitimate (but "free") app:

These things are just pathetic scams.

tonkalorlor wrote:

but I never visited an adult websites so that’s even weirder

Your persistence in assuming truthfulness from spammers, scammers, grifters, and con-artists is positively quaint.