The Prius architecture is always terribly described, but it's effectively a differential. Always in mesh, no gears to shift. The speed of the wheels is varied relative to the speed of the ICE by shifting power between generators and motors.

So, to the driver it feels like a CVT with very fast ratio adaptation, but mechanically there's no ratio being varied. It's all fixed gearing, and the variation is done with motor RPMs and torque.