On Xbox’s news blog, Xbox Wire, they gave an explanation of how their Smart Delivery Program will work to bring your Xbox One games to the Series X. It boils down to three scenarios.
First, if a game is attached to your account through either a purchase on the Xbox One or Game Pass then you can play the enhanced version on Xbox Series X on launch day.
Second, If you buy a game post Series X that is available on both systems then you gain both versions. So if you play on the Series X then you play that version but if you play on the Xbox One then you play the Xbox One version.
Lastly, if the game is a third party title on Xbox One then you can still play it on the Series X BUT it won’t be enhanced. You will need to purchase the enhanced version separately. This is still backward compatible but without the free enhanced perk for Xbox Exclusives.
As a reminder, PS5 backward compatibility will be limited to most of the top 100 selling games on the PS4.