1. In flight, the main engines have generators.
2. When main engines are off and no ground equipment is available, there is an auxiliary power unit (APU) mounted in the tail cone. This uses the same fuel as the main engines, but only generates electric power. No thrust.
3. At a gate the aircraft can be powered from the city’s electric grid.
4. Away from a gate there is a portable generator known as a ground power unit (GPU). Much like the APU except mounted on a towable cart instead of the aircraft.
5. If engines and APU stop working in flight, there is a little windmill called the ram air turbine (RAT) that is deployed from the underside of the fuselage. It generates minimal electricity from forward air speed.