Weirdness with external monitor on Catalina 10.15.7

I got a new "refurbished" Mac today, 2017 13.3" Mac Air with Catalina 10.15.7. It's gorgeous. My 2012 11" Mac Air has been dying for a long time; it was on High Sierra. I transferred everything this afternoon between Macs.

I plugged in my HP monitor and it shows the Catalina graphic but no other activity. If I close the lid of the Mac, it shows the app I'm using. What??? So strange!

I use my 32" monitor to folk dance with hundreds of people on Zoom, it's a primary activity. What am I going to do now? I have read several threads but did not see my specific weirdness. I'm just stumped.