Microsoft has been more ambitious with backward compatibility. The SERIES X will play original Xbox, Xbox 360 and Xbox One games. That's four generations of games, stretching back to the first console's 2001 launch.
But it doesn't include literally everything. Microsoft has a list of the games that play on its new consoles: 568 games from the Xbox 360 and only 39 games from the original Xbox library are currently backward compatible.
Old games get a visual boost on the Series X and Series S, by way of high dynamic range graphics.