Beyond a certain point in descent to the surface it will be all on-board sensors. The maneuvers to put the vehicle in the right attitude are based on a ton of orbital mechanics modeling and simulation to program the thruster burns and torquers. No doubt with fine tuning by ground command if necessary based, again, on on-board sensors. I have no doubt Bennu was imaged on approach to fine tune the orbital dynamics modeling, in particular its morphology and rotational state. Something similar was done with Deep Impact during approach, but that was tougher because instruments had to image through the dust of the coma. A comet boils off an amazing amount of debris embedded with the gas due to exposure to the sun. In some respects Bennu will have been more straight forward to image for update of the modeling.