How do I remove pop-up saying viruses affecting iPhone?

You don't have any viruses on your phone, it is a scam message designed to get you to purchase unnecessary software or phish for personal information.

Read more about it here -

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

How to get rid of them? Stop visiting the websites where they pop up.

Settings, Apps, Safari and clear all history and data. If you have other browsers, clear all cookies

If necessary reset network settings.

Your friend was visiting web sites known for displaying scam messages or pop ups.

You might not be aware that the web site was paid to show the scam messages that you are now seeing. You can be assured that the messages will continue. In fact, your contact information has very likely been sold to other scam sites.

Other bad things can happen when you let a "friend" use your phone, so you might want to think twice before you offer a free loaner again.

Annoying and mostly-harmless popup advertisements aside, I’d wonder about what other issues might now be awaiting here too, including whether you’re now banned from Instagram or Snapchat or some other service (as borrowers can borrow devices just because they expect they’ll get permabanned), and also whether your passwords or accounts have been copied or compromised.

Contact the carrier directly, as the carrier’s Thales / Imperva firewall is blocking your access.

This firewall is intended to block bots and denial-of-service attacks, but its own in-built or added rules can misclassify traffic.

Various rules loaded on the Imperva firewall has been generating this error for over a decade, and I doubt Thales / Imperva and the carrier will explain why.

Per one of the related replies, it’s probably a rule implemented by the carrier, too: The website is protected by an Imperva firewall which has rules in place to try and block bots and denial of service attacks. These rules often trigger on legitimate users for reasons only the [site] know (but as someone who's used Imperva extensively in the past it's almost certainly a rule they've put in themselves not one of the built in protection rules).

This is not an indication of malware, viruses, or any other issues locally.

As a SWAG, the Thales / Imperva firewall is (mis)detecting and blocking iCloud+ Private Relay, but there are other potential causes of the carrier’s rules or generic rules causing a problem for the carrier.

If the problems contacting your carrier persist, find a different carrier.