I have often thought about how these 'sensors' work considering that there is only 2 of them 1 above & 1 below and no less than 8 more basic camera 'sensors.'
Forward, backward downward cameras : they are 3D cameras and work just like your eyes. If you walk towards a wall, you will stop before hitting it right ?
sideway cameras : similar to the cameras above but they are not 3D. I have been suspecting that they detect distance by using the focusing mechanism of ordinary cameras.
IR sensors on top and bottom : The sensor transmits an IR beam which is reflected back to the sensor by the obstacle. The time of flight of the IR signal tells how close the obstcale is.