How aircraft engine function?

with jet propulsion,

Air is taken in at the front, than compressed by an compressor. Fuel is added tot the air and ignites. This drives the temperature higher, making the air expand and increase the pressure. After that a turbine takes some of the energy of the flow, decreasing the temperature and pressure and converts the energy into work. the work than is used to power the compressor. The exhaust provides a thrust for the airplane.

with a propeller,

an engine drives a propeller which provides thrust for the airplane