It is similar how planes starts. First small turbine engine (APU) starts. When APU fails to start, they use specialized airport cars with compressed air. Then t

It is similar how planes starts. First small turbine engine (APU) starts. When APU fails to start, they use specialized airport cars with compressed air. Then the turbine helps to start first main engine. Then the main engine helps to start second main engine. Repeat until all main engines are started. (https://www.youtube.com/watch?v=GzhdxSsoT0g https://www.youtube.com/watch?v=GzhdxSsoT0g)