What makes an Olympian god an Olympian god?

Suddenly this question came to me, well we all know who the gods of Olympus are but what exactly made them these gods?

At first I thought of the obvious, which is that one of the requirements is to be the children of Chronus, BUT as we well know Hades is not a god of Olympus, even though he is the eldest son and is ruler of the Underworld in the same way that Zeus is of the sky and Poseidon is of the seas, not to mention that in some versions Hestia is not an Olympian, instead she is her nephew Dionysus.

another idea is the children of Zeus, which also makes sense BUT Zeus has many illegitimate children who are gods, for example he has children with Themis and none of his daughters have a place on Olympus but they allow other illegitimate children like Hermes, on the other hand not all of Zeus' children with Hera are Olympians like for example Eileithyia, but Ares is and the gods hated Ares.

and there is also the case of Aphrodite in which in some versions she is the daughter of Zeus but in others she is a goddess even older than him, how in this version did she become a goddess of Olympus?