That I do not know.
My guess would be there’s some sort of electronic controller which directs power to the drive-motor to the extent it’s needed to respond to the “gas pedal”, and whenever excess power is available charges the batteries.
Or, maybe the batteries are disconnected entirely when the aux engine is running.
The latter would be far simpler/cheaper.
If that's the case, then it seems like any quoted limit (e.g. "40 miles") is meaningless since it totally depends on what kind of driving you're doing.
Anyway, that makes more sense than what I envisioned: you drive 40 miles, then pull over and fire up the engine to charge the batteries for an hour. Yuck.