Contributing to the discution, at the begining, it may sound unfamiliar but big jet engines use pressurized air to start up. The air is ducted to a device on the engine that turns air pressure into movement of the aircraft engine itself. After a given rpm the engine ignites itself and becomes self sufficient. This pressurized air may came from APU, aircraft engines or an External Power Unit.
Once APU or aircraft engines (aircraft engines are also source of pressurized air) are available on their rated rpm, they can feed other systems which rely on pressurized air like anti-icing, aircraft pressurization and climatization.