Well, there is a way to toggle Universal Control ... which I've found useful while troubleshooting Universal Control,
Add Displays to the Mac's main menu bar:
System Preferences -> Doc & Menu Bar -> Display -> Show in Menu Bar always
A new icon will show up on your main menu bar which links like a miniature split screen when Universal Control is working, and like a miniature monitor when it isn't.
If it isn't working, but potentially _could_ work, click on the miniature monitor and if you see and you see "link keyboard and mouse to <name of your iPad>" with a little dark monitor icon below that, then click on it that; it should turn green and look like a little split screen. Thus toggling it on (without having to try to drag the mouse over past some edge of the monitor).
If it is working, the little icon in the popup should be green. Click on that to toggle Universal Control off.
Note that in Display preferences, for your iPad you must have either chosen to Use as: "Linked keyboard and mouse" (which is Universal Control) _or_ Use as: "Extended or mirrored display" (which is Sidecar). Whichever one you choose will foreclose using the other.