How does Aircraft function its Electrical system and ACU when the main engines are off?

The airplane’s electric system and ACU (Air Conditioning Unit) run either on GPU - Ground Power Unit or APU - Auxiliary Power Unit.

A Ground Power Unit (GPU) can be either a fixed (connected to Aerobridge) or mobile unit. These power units usually consist of a generator powered by a diesel engine.

APU is a small, jet engine which can be started with battery power.

Its main tasks are to provide:
• Electric power on board
• Compressed air to start the engines
• Compressed air for air conditioning

APU stands for "Auxiliary Power Unit." It's a small engine or power source located in the tail section of an airplane that delivers auxiliary power for different functions when the main engines are not operating. 

The APU can power systems such as electrical generators, air conditioning, and hydraulic pumps when the aircraft is on the ground. It helps enhance the aircraft's operational flexibility and efficiency.