Almost.If a developer wants, they can create one SKU or their title across Xbox One and Xbox Series X. You can purchase the SKU and depending what system you are on, it will work as intended on that system. It's basically Play Anywhere in the sense that one purchase by you will net you a playable game on both systems.
Any One SKU will play on the SX.
If a game is Smart Delivery enabled, buying a One SKU will also get you the SX SKU, and vice versa.