After watching the behind-the-scenes for the David-Leitch-directed "Snowbrawl" action short, I scrubbed through it to get a better look at that camera rig. The iPhone 11 Pro Leitch is using is ensconced within this sort of cage:
Thank you, freeze-frame. I looked it up and learned that the BeastGrip Pro, as it's called, is the direct result of a Kickstarter campaign. In 2013 Chicago-based inventor Vadym Chalenko, armed with a 3D printer, pumped out the original BeastGrip:
He was seeking $25,000, and netted nearly $40,000. That was enough to help him get BeastGrip off the ground as a company. In 2015 he launched another Kickstarter, this time for his improved BeastGrip Pro, and this was an all-out smash. Seeking $50,000, they netted a whopping $265,897.