Why can a spaceship fly around the earth for so long? What is used as fuel?

Obviously, launching from the earth needs fuel.

After reaching the predetermined orbit, you don't need to push it, let it fly by itself. Because there is no air in space, that is, vacuum, and there is no resistance, you can fly without pushing a spaceship.

But when changing the orbit, such as flying around the earth before, and now flying to the moon, an engine is needed at this time.