Liquid cooling or 'thermal spreader': check out the S7 edge heat pipe insides (video)
There you have it, we definitely don't have liquid nitrogen running in your phone, being pumped by tiny goblins to swoosh elegantly around the CPU in clouds of frozen mist. What we do have, however, is flatter copper piping with a minuscule amount of liquid, often just water, that evaporates when it hits the CPU area, takes the heat away with the gas it forms, and condenses back into liquid when it runs further down the pipe, like on the scheme here:
Samsung itself calls it simply a "thermal spreader," instead of liquid cooling, though technically we likely do have a drop of liquid in there, and it's made on a very, very compact scale. Want to see how it actually looks like from the inside? Well, the video below comes to the rescue, from about 4:30 in, and you also get to see the other set of tiny goblins - the ones that are trying to hold the camera steady when you shoot video, or the so-called optical image stabilization. It's fascinating, check it out.