I originally got the idea for this setup from a YouTube video. I've since seen several creators use a similar concept and installation method, but I can't remember which video I first followed, so I can't credit a specific creator. This is my version of the setup; other builds use different phone holders and wired or wireless chargers.
They have a little flex to them. I can tighten them down more so they don't move at all, but I like being able to shift the arm left and right.
They're not screwed on, they're bolted on, in between the dashboard pieces. That's how committed I am to this system. I put a YakAttack GearTrac track there and bolted it down. One side is for me and the other side is for my wife.
It's pretty straightforward. The track ball sits on a T-bolt that slides into the slot, and as you turn it, it tightens and locks in. Then you need the double socket arms to go on the balls. They come in a few sizes, short, medium and long, and the arm puts pressure on the ball at each end.
To tighten it, you turn the knob clockwise until it's snug. Mine is set so it's as tight as I want it at that point, and it's not going anywhere. My wife can easily throw her phone on there, and there's still enough give to adjust it.
It takes a little tinkering to get the tension exactly the way you want it. You just keep rotating it and tightening it. Once it's set up, it's easy.
On the driver side I use the RAM Quick-Grip phone holder, which gets its own video. The whole setup is really solid. I've seen people mount these in all kinds of places, and I'm looking to add another ball because they're that handy.