I’m not super informed on this part, but my educated guess is basically gravity+friction+viscosity.

I’m sure you understand viscosity in the example of, say, honey, but it can be harder to understand for less (lmao) viscous fluids like water or air, but the same principle applies. Imagine the atmosphere was made of honey and not air. The earth would rotate and the honey just on the surface of the earth would match that rotation, then the layer of honey just above the layer touching the earth will start moving, not as fast as the bottom layer (yet). Then allow the system time to equilibrate, each layer slowly having reached its max rotational speed through the combination of gravity, friction, and viscosity. As the honey gets thinner the effects of viscosity will be less pronounced, thus less friction on surrounding objects like satellites and shit.