They are indeed desk mounts. You can clamp to the back lip of the desk, or go through a cable management hole, or drill your own hole in the desk. I would not recommend this mount on a glass top desk.
I've got a Dell U2718Q and S2716DG mounted on each mount. They're ~12# each. For a while I had a 16# monitor on one of the arms. The tension is adjustable for heavier monitors. I've had them for a couple months and I probably move them (to get to cords, move random stuff around) about once a week. They're great for the price.
I'm sure there's more expensive and robust arms out there, but for $20 each I'm quite happy. They come with cable management and USB extensions. My cats have crawled all around the monitors and arms and they've yet to fall off.
I do feel odd about having $900 worth of monitors on $40 of arms, but they function quite well and do not sag.