This simple app opens the main camera and displays it via fullscreen without any bars in full resolution.

It is not intended to take pictures or actively stream to web.
You can connect the phone via USB and use scrcpy to stream the feed to your computer and then use OBS to record it or create a webcam out of it.
This is a very easy and cheap way to create a proper USB based webcam out of your Android phone.

And the best: There is no recurring cost or watermarks.