Windows 10 takes entire screen

I'm running Parallels 15 on a 27" iMac with Catalina OS 10.15.4. I do not know how I got into this situation, but when I run Windows 10 I can't make the Windows window share the screen with my other Mac windows like Word, Excel, Outlook. I can toggle back and forth between Windows 10 (filling the entire screen) and my usual OS multiple tasks, but the control in the upper right-hand corner that I would ordinarily use to reduce the Windows 10 window isn't there. There must be a switch somewhere in the vast menu of Windows controls.