The Greek pantheon is unusually well documented and unusually badly balanced, because what survives is mostly poetry, and poetry had reasons of its own.
Homer and Hesiod are the foundation, and the Greeks said so themselves: Herodotos credits the two of them with having given the Greeks their gods' names, honours and shapes. That is a remarkable admission. It means the canonical account was understood, even in antiquity, to be a literary settlement rather than a revelation, and Hesiod's Theogony in particular reads as an argument for one arrangement of the divine world among several available.
Tragedy then does something different again. The Athenian stage takes the same gods and puts them under moral pressure, and a reader who takes Euripides as a straightforward report of belief will end up describing a religion nobody practised.
Underneath the literature is a much older and much drier layer. The Linear B tablets from Knossos and Pylos, administrative records rather than myth, already name Zeus, Poseidon, Hera and Dionysos in the Bronze Age, centuries before Homer. They tell us these gods received offerings; they tell us nothing whatever about the stories attached to them.
The cult record fills part of the gap. Inscriptions, sacred calendars, dedications and the physical remains of sanctuaries show a religion organised locally, by city and by deme, in which what mattered was the correct performance of a rite at a particular altar. Delphi and Olympia served all the Greeks. Everything else was somebody's neighbourhood.