Alternators and generators are sorta the same because they both generate and produce electricity needed for the vehicle to run.
But alternators are more efficient because they conserve energy and only use enough energy to keep the engine supplied with electricity and keep the lights, radio etc on.
A generator keeps producing all the energy whether it's needed or not so there's a lot of wasted energy when the electricity is used from a generator.
Just like with the electrical grid no matter if you turn the electricity off in your home or not the generator is still running and producing that electricity.
So shutting your lights off is not gonna save any earth resources.
The alternator only outputs though enough voltage to keep the engine going and the lights and everything else going when needed.