Do the games have to be released at once on the X and S series?

It could be the case in the future. I couldn't possibly say. We aren't Xbox employee's.

Currently, as I understand it, the policy has been to release both the Series X and Series S versions at the same time. That could change in the future though.

I have. You do not have a Series S. You have a One S. There is the Xbox One family of systems which includes the One, One S, and the One X. Then there are the newer consoles which are the Series S and X, which are in the picture I posted above. That game has not been released on the One which means it will not work in any console of that family including the One S you have. The game was released for the Series S and X.

Spencer said all generations of games will work on the Series S/X and Microsoft Studio games will be released on Game Pass day 1 for both the One and Series S/X. The Medium is not a Microsoft Studios title so the publishers/developers have decided to release
it for the Series S/X.

Hello Rhmd, to do this, you need to set the Series S as the home Xbox, then make sure both your account with the games on the Series X, and his account are both signed in on the Series S. Once you do that he will have access to all of the games on the
Series X from his Series S.

They released that game as a One version and as a Series S/X version. The Series S/X version will only work on a Series S/X.