I searched and searched for the answer to "How to turn off Center Stage on a Mac". Eventually I figured it out. I’m on an iMac (not a Studio Display). Since we can’t upload a screen capture here, I’ll describe turning off the Center Stage feature in detail.
With Zoom open, at the very top of the Mac screen (in the area Apple calls the "Menu Bar")… For me, the date & time is the furthest to the right. To the left of the date & time, is an icon for Control Center, icon for Spotlight search, icon for WiFi, icon for Bluetooth (your icons may vary depending on your System Settings). Then, the Zoom icon (a little blue circle with ZM in the center). To the left of THAT is an icon that represents a camera — it has a green background and a graphic that's supposed to look like the side-view of an old-fashioned movie camera facing to the right… a white square with a small white triangle on the right side. THAT is the green camera “button” people have been referring to here. When I click on it, a drop-down menu appears. At the top is the Zoom logo with zoom.us next to it. Below that is a thumbnail of whatever your camera is viewing (probably your face). Below that are the words "iMac Camera". And below that are menu choices with icons:
Center Stage
Portrait
Studio Light
Reactions
Background
Desk View…
If you click on the icon to the right of the words “Center Stage”, it turns off (and on) the Center Stage feature. I hope these details help someone.