Sounds like they were using one of many Apps out there that will black out a screen and hide the dock when you’re working on one document. I’m sure it was configurable to allow the program you want to use the front window, but easy to return to the full screen.
I looked around and found quite a few that are free and some that are 99 cents to $19. But a free one called Isolator works great for me. I downloaded it and had it running in a minute or two. Download it and open the disk image and then copy the Isolator icon to your Applications folder. Then double-click that and isolator will give you a little square icon in your menu bar. In that is a menu where you can start Isolator each time you start your Mac, select if the Dock still shows when everything else is blcked out, quit Isolator when you’re done, and more.
See a screenshot from the Isolator site below. Download from Isolator. I might start using it all the time myself.