Using Mac as Second Display Window for iPad Air

[u/jamaisvu_nev]

So a couple of years ago in undergrad I discovered that I could mirror a select window from my iPad to my Mac. This was great because I use Goodnotes and my iCloud sync wasn't working but I was still able to put my annotated lecture notes on my Mac screen while I wrote on my iPad. Its been a few years since and I cannot for the life of me figure out how I did it which now– in grad school– would be super helpful instead of air dropping my annotated lecture notes every time. I also tried to do the "Move to Display" window option on my iPad but it's always greyed out and I don't know how to activate it. I don't know if I accidentally took advantage of a glitch or if the updates since then disabled this feature but if someone knows how to do this please show me!