The planet of the symbiotes is called Klyntar (which is also the name of the symbiotes' race). It is an artificial planet created to imprison the symbiotes' creator, Knull.

What planet is symbiote from?

Klyntar (the symbiote word for "cage"), also known informally as the Planet of the Symbiotes, is an artificial planet, located in an uncharted sector of space not on any known maps; though Thanos managed to obtain its coordinates and provided them to Wraith.

How did venom get to Earth?

In the comics, Deadpool and then Spider-Man encountered the Venom Symbiote on a planet called Battleworld during the “Secret Wars” storyline. Peter Parker wore the Black Suit Spider-Man costume and brought it back to Earth, eventually rejecting it outright for fear of its sentience.