you haven’t seem them because your gym sucks. it looks like this ( more after the video ):
between my two gyms i have four of them and all four are different, but i only use one in each gym.
to develop legs — sprints, jumps, all sorts of agility drills, kettlebell swings and snatches of any type you like ( bar, dumbbell or kettlebell ). this won’t give you the biggest legs but it will be structurally healthier than if you did mass exercises like leg extensions. unless you are a professional bodybuilder i don’t see why a man would want the biggest legs possible.