After you scan and process the file using 1 of 3 techniques, it gives you an option to “Edit”.

Then use “Crop” to isolate to what you want.

Edit: also, FYI that the different Processing Modes use different algorithms. The “Area” and “Detail” are the best. When your scan looks odd with one, click cancel and try the other.

Lastly, the better you are about maneuvering your phone while capturing, hitting all angles, not casting shadows into the area, and so on, results in a better outcome.

One of the coolest parts is if you choose Share>Export Model>USDZ then you can send it to another Apple user via iMessage and they can see the model instantly (with no app required).