received a "critical threat" message stating my device (iphone 8) has been infected with 19 viruses and says I must take action immediatiely.s

Unless you've jailbroken your phone, you have NOT been infected with anything.

This is what is called a phishing scam, engineered to get your personal information and money for something you don't need.

Do NOT give them ANY information. Do NOT pay them anything. They are scum. Thieves. Bad people.

See this link --> Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

All such messages are scams, presented by criminals. Close the offending page, and avoid sites that are involved in such bogus advertising tactics.

It’s a scam to try to steal your identity. Ignore it.

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