Sounds to me like it’s part of the screen zoom feature. You can set it to zoom the whole screen or just a part of it (Picture-in-Picture) which is like a rectangular magnifier box that moves with the cursor. You can set the parameters Accessibly -> Zoom and select full screen, split screen, or picture-in-picture… it’s all there. You can turn off zoom completely in the main Zoom window.
My choice is the picture-in-picture box, triggered by a keyboard combination. Very useful for these tired eyes.
(My description is for the newer System Settings interface. I don’t remember exactly how to get there in the earlier versions of macOS, i.e. System Preferences, before it became System Settings. I think that was Monterey and older. I think Ventura started System Settings.)