Virus

It must be a notification from the last app you installed before the problem started.

Go to your settings, find the app and turn off notifications from that app.

If you don't see those popups again, delete the app.

Then it could be the sites you're visiting. Do not tap on any of the popups as they are all fake. Also, ensure that you do not install the app it's prompting you to get.

It's highly unlikely that your iPhone has a virus.

Where does this popup show? Is it on Safari or is it in any app?