How does the Macbook check the ambient light to determine the screen brightness, does it have a separate ambient light detector buried under the screen somewhere or inside the camera? (if the camera is not used for this I mean which would have been the best thing to do, but would require the camera to grab frames now and then without the LED flashing)

In this vein, apps on your phone likely have unrestrained access to the photosensor, and researchers have figured out how to take low-resolution photos with it: https://news.mit.edu/2024/study-smart-devices-ambient-light-...

Macbooks have a dedicated ALS (Ambient Light Sensor). They don't use the camera.

There is a third little dot near the camera you can see with a flashlight and that's an ambient light sensor.