I received a pop up stating my iPhone has been compromised by 13 Trojan viruses

Your phone cannot get a virus from any source. However, scam alerts that your phone has a virus can appear several ways; from visiting sketchy web sites, from apps that don’t vet their ads well, as email, texts, and as calendars. Some sites trick you into installing a spam calendar by offering you something for free. That’s why deggie asked if it was in a calendar, because there are specific instructions for removing spam calendars.

It’s a scam to steal your identity or your money. Ignore it and delete it. See→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Was this in your calendar or just a pop-up? If the latter delete it and don’t worry about it.