There do exist serial hybrids where the engine produces electricity while maintaining the ideal RPM, and then is connected to a battery+electric motors.

However in practice the vast majority of hybrids do not use this approach and have motors that vary RPM with road speed (depending of gearing of course).

The common case of maintaining ideal RPM is the CVT, which most folks dislike, so much so that some models have a switch to pick how many fake gears you have to break up the boring drone of a constant RPM engine.

BTW, the chevy bolt was advertised as a serial hybrid, right up to the day it shipped.

I believe the most common serial hybrid today is an EV with a range extender.