How a helicopter engine works?

In a very similar way to ordinary aero engine. There are internal combustion engines which work on the induction, compression, power and exhaust system, linked through a gearbox to a shaft which drives the prop. Or the jet version which works by hot gases turning a turbine, which has a shaft running through the middle, again linked through a gearbox to a shaft which drives the prop. This is called a turboprop.