Its all done with "smoke and mirrors"
But really, so many things factor into it; lighting, humidity, ambient temp, etc. I ve read that adding distilled water to high VG adds a thicker cloud.
One big thing is inhalation technique and lung capacity and a lot of practice.
I used to vape with a basic Boge in a carto tank, 50/50 liquid. I had some pretty decent clouds ( at the time) and people would be surprised by it. I gave the setup to my brother in law and on his first puff he filled the room with such a cloud that I could not see his face. He looked like a cartoon, it looked like it was coming out of his ears, nose and mouth.
I said damn, why cant I do that? He is a hardcore euro smoker and they "SMOKE" he did not even gag.
Many of the reviewers , especially when the "sub-vaping" started being the thing, would be doing their " clouding vids" and yes, the cloud was giant, but almost all of them would be red faced and gagging or hardly be able to talk after their performance. Those that are doing it now have conditioned their throats and lungs . These guys and some gals have spent 1000's of hours in front of the mirror practicing and pounds of wire getting the build just right.