I don't know where to submit screen captures to be viewable by anyone without creating accounts and such but here is a URL to the app the ad in this window is advertising for: https://apps.apple.com/app/id1525556960?mt=12

The first screenshot is what I see on my screen, except with a message at the top saying "Mirror My Screen (formerly MKMirror) - Free Download expires soon" and a "Download on the Mac App Store" button at the bottom.

Why isn't there any way to close this window without having to click this button in particular? And to be more clear, I cannot close it with Activity Monitor or Force Quit it in any way.

At first, I wasn't sure if it was a browser popup window and clicking anywhere in the window doesn't show me what macOS app this belongs to. For all I knew, it could've been malicious in nature and I wouldn't have known until clicking the button.

Finally, this window appeared after installing updates for macOS Catalina, I've mostly avoided going to Big Sur for now.