You don’t start a plane like a car, simply because an airliner has no ignition key. To start the engines, pilots must follow a triple stage procedure.
On long-haul jets, such as the Boeing 777 or Airbus A350, everything begins by turning on the auxiliary power unit (APU). In the cockpit, pilots turn the "APU" switch to "On". The APU then starts and provides the pneumatic energy required to turn the engines over.
The second stage is then to turn the "Start / Ignition" button from the "Norm" to "Start" position. This action opens a valve allowing air to be injected in order to rotate the fan blades; only when the mechanism inside the engine is set in motion, and rotating at a certain speed, can the engine be supplied with fuel.
Third and final stage is the fuel supply to the engine. For this, the captain pushes the "Fuel Control" lever (located below the throttle) from "Cutoff" to "Run". At a certain rotation speed of the engine compressor, the fuel valves open. The compressed kerosene-air mixture then ignites, after which the motor then operates independently.
This operation is carried out for each engine, generally one after the other, and takes about one minute.
This jet engine automatic start procedure is the most common. During the three stages, the onboard systems and computers carry out the start-up of the engines in complete safety. The pilots, themselves, supervise the operation via a monitor screen in the cockpit. A ground engineer is also in radio contact with the crew during this time.
Whilst there is no ignition key to start an aircraft, just remember that there are no door keys either.