They watch snipped video's, pointing out tendency's and what you can do to exploit or stop them. They're not watching video's showcasing a player deking through a team or making no look passes.
For instance, a video on McDavid would show his much practiced 3 point wind up the ice without the puck, in which he opens to take a pass 3 times. 1st open to take the puck up the ice, 2nd for a quick play attack, 3rd which is the variable dump/break past close side D or pace his skating for far wingers support. A video on Crosby would show him turning back and forth deep in the offensive zone, waiting for opponents D formation to break and then passing it to his Dman for a shot.
There's no point in wasting time watching talented players make plays naturally based on the situation, because guys like McDavid/Crosby can make up plays up the fly. So you pay attention to things they've practiced and their tendency's, so you can actually create counters to it.