Xbox One currently has 25 "X enhanced games" Xbox 360 games which are rendered at 4K on Xbox One X, a nine-time increase over 360's 720p resolution. Compatible original Xbox games have their resolution multiplied to the nearest native value on all Xbox One models, but are improvements to the 360 titles X-exclusive? According to this Forbes contributor article Red Dead Redemption runs at 1080p on Xbox One S which may suggest that at least the S model benefits from the X-enhancements.
Also, do all the other 360 run at the original resolution (720p) on every variation of the console?