Actually it was Baer and the 1972 Odyssey that had the first cartridges.
Not cartidges, cards. The cards on the Odyssey were just a chunk of plastic with strips of aluminum contacts. No electronics at all. Depending on which contacts were present on a given card, the analog circuits in the Odyssey would do different things.
The entire game engine was analog. The continuously varying voltages in the transistor caps deflected the cathode ray tube.
It was an elegant design, but had nothing in common with modern digital video games.