At the heart of the Atari 2600 was the TIA video chip, designed by Jay Miner, well known for his later work on the chips for the Atari 400/800 and the Amiga.
Who manufactured the TIA? As an in-house custom design, it couldn't be simply purchased off the shelf; I haven't seen any mention of Atari owning a chip fab, and hiring such wasn't as routine then as it became in later decades. Did they make a special arrangement with a chip company?