To setup a USB document camera to share it in a Google Meet, you'll want to open the camera application on your computer. Hit the start button and type "Camera". When the app is open you can click the icon in the top right to select the different cameras if your document camera does not show up first. If it does you can leave it and move on to the next steps. Keep the camera app open.
Open a Google Meet. In the bottom there is a "Present Now" button. Click it and select "A window". Find the Camera app in that option. Share it. You should now be showing your document camera and your web camera at the same time to do a presentation.
To check your web camera settings click the three dots in the bottom right and click Settings. Click Video and you can select which camera is showing as your web camera if the correct one is not. You can also set the resolution for incoming and outgoing video. It would be good to at least set your Send resolution fairly high. Receive is up to you and your internet connection.