What is the function of the hole at the rear of a plane? I searched online but didn't get a satisfying answer.
This one? It's the APU(auxiliary power unit)'s exhaust.
An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion.
The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. Before the engines are to be turned, the APU is started, generally by a battery or hydraulic accumulator. Once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft's main engines.