It's hard for me to fathom how they do it too. But as with anything, I think it probably goes without saying that after you do it quite a few times, the fear either goes away or become controllable because of (1) comfort with being up high, and (2) confidence in your ability to function up high. I know when I was in the Marine Corps, in boot camp, they had you run various obstacles in obstacle courses, some of which involved heights. They referred to it as a "confidence course." And it very much was. Quite of few kids right out of high school are afraid and even downright terrified on some of those obstacles. You make it over the first time because you have drill instructors screaming at you, and you are more terrified of them than anything else. Then, having done it once, it is easier and less scary the next time. Then, for me, fast forward two years when I am doing jungle training in Okinawa, and we have to do things like zip line over water where the line is 20-50 feet up, or rappel down the side of a cliff. I never could have even thought about that prior to boot camp. But having done those obstacles back then helped with the initial step. And then with the jungle obstacles, you have someone talking you through how to do it, before and during. So I can definitely see how comfort and confidence can grow. Still not something I have any desire whatsoever to do. But I at least get it.