Depends on the bike. On some bikes the motor is mounted in the rear wheel, in which case there's no gear between the motor and the wheel. On other bikes the motor is mounted between the pedals and sent to the rear via the chain, in which case shifting works as you expect. But the latter style (a.k.a. mid-drive) demands custom frames (because mid-drive motors are nonstandardized), which increases costs and decreases repairability. In contrast, rear-wheel motors can fit on literally any frame, so they're much more accessible.