@wetbeardhairs has a good explanation, but i wanted to highlight the single key difference.

Extended Range means the engine doesnt connect to the wheels. All the engine does is spin a generator. No trans or torque converter. Its not really part of the powertrain anymore.

Just a bigass generator under the hood. The engine powers the HV lines on the car, so it can charge the battery or power the traction motor.

Electric generators/motors are pretty efficient, so you dont actually lose much power in this process, and the systems controls on series HEVs is easy. Just turn on the engine if the battery is low