What is causing "Virus Detected" popup notifications?

I am recently getting these pop-ups on my MacBook Pro (upper right corner) about detected viruses, my Mac OS system is in danger, etc. My understanding is that the MacOS system is impervious to viruses (?) but I'm not sure how these pop-ups have recently shown up or how to get rid of them. And, if they're doing this, is it possible that my system IS infected?

Thank you!

[Re-Titled by Moderator]