iPhone 7 got a pop up saying there is a virus

All such messages are scams from criminals. Do not interact with the message or the sender. Do not download anything based on a popup message. Close the offending page and avoid sites that are involved in such bogus advertising tactics.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

If popup you are referring to is notification from Calendar app, then you have subscribed to scam calendar account. Go to Settings > Calendar > Accounts, then check each listed configuration and remove one that is associated with those scam events and account.