What is a demigod?

No joke, im honestly confused. Maybe ive missed it, but i dont get what capabilities an average demigod is supposed to have.

Dionysus started as a demigod, gained his powers and became a god somehow.

Heracles is still a demigod, somehow, so is he immortal? Or ageless?

For that matter, are we becoming a god? Our fate is evidently to die, but if that doesnt happen, do we live forever? And how come our powers are just Demeters when Heracles and Dionysus's powers dont have anything to do with Zeus?

No joke, im kind of lost. Can someone elaborate?

(Note. I know the mythology answers. Im looking for strict in universe answers.)