For better picture quality and to avoid accidentally showing your surroundings on your Android device, you will need to connect a USB camera to your device. As soon as you join a meeting, you will be able to choose the video source as your camera.
It is only after you have joined a meeting that you will be able to select your USB camera. It is possible to use any video device that is compatible with USB video device class (UVC protocol) in meetings.
Select the USB camera when you are in a meeting by tapping Video > Camera settings.