The Greek gods are a family, and the family has a history of violence. Hesiod sets it out plainly: Ouranos is overthrown by his son Kronos, Kronos by his son Zeus, and the threat of a third overthrow is the reason Zeus swallows Metis. Succession here is not inheritance. It is seizure, and the pantheon's stability rests on the last usurper having successfully closed the sequence.
Zeus rules, but his rule is bounded in ways a sovereign in other traditions is not. He presides over an assembly that argues with him, he is deceived by his wife and his brother, and above him sits an impersonal allotment that even he is not shown reversing. The Greek arrangement is a monarchy with a constant undertone of contingency.
The roster is nominally fixed and practically open. The Twelve are a real cultic category, but the lists differ, Hestia and Dionysos trade places, and the whole set sits inside a far larger population of gods, daimones, nymphs and heroes with local cult. Foreign deities entered readily, and were usually identified with a god already held rather than added beside one.
The door from mortal to immortal is genuinely open, which is rarer than it looks. Herakles is burned out of his mortal part and received on Olympos; Asklepios, Dionysos and Ganymede each cross the line by a different route. Below full divinity, hero cult gave hundreds of dead men and women real, located, sacrificial worship. The line between the human and the divine in Greek religion is a graded slope rather than a wall.