How does the Audi A2 save energy?

I'm interested in purchasing an Audi A2, but I've heard it has some energy-saving features. Can you tell me what those are and how exactly they work?

The Audi A2 uses a lightweight aluminum body construction to reduce its overall weight and increase its fuel efficiency. Additionally, it features regenerative braking, which converts the car's kinetic energy into electrical energy to help power the vehicle.

The A2 also has a start-stop system that automatically turns off the engine when the car is idling, helping to conserve fuel. It also has an ECO mode that optimizes the engine and transmission for better fuel efficiency.