Depends on the configuration. If they are equipped with a CVT+electric engine on the shaft, then that's the case: they rev to the sweet spot (which on gas engines is 4000-5000 rpm on acceleration? Depending on expected engine load I assume) and the speed is controlled by hydraulic force applied on the CVT.

With other hybrids: depends on the generator they have installed, but it matches the consumption in amps by the engine in order to "go" if it is not directly coupled with the transmission, or they just downshift to accelerate with help of the electric engine.

I am assuming a lot here: Toyotas (specially RAV4) mount CVTs among others, assuming pure electric generator by the ICE or coupled to it... So it depends a lot on specific configuration.

Even CVTs will rev the engine if you floor it, no? They typically allow for wanting to be in a few sweet spots. I prefer to keep it near the max efficiency speed. That said, my wife is a bit more aggressive and spends more time in the max power mark. It has a predictable change on the MPG reported by the vehicle.