How can you sell digital codes for Xbox Series X/S games?

I am thinking of buying hogwarts legacy as a digital game but it is sold out for Xbox one so should I buy the Series X game and put the code in the Xbox one Store?

No. The series X/S version of the game will only work on the Series X or S.

The only way that would work is if it's a code for the digital deluxe version of the game (which comes with both the series x/s and xbox one versions of the game)

Game also sells digital games via Xbox Codes. You might be able to get FIFA 21 that way, and then you will be able to get the digital copy for the Series S at no extra cost.

I'm sorry. Xbox Series X games work on an Xbox Series S console, as long as it's a digital version because the Xbox Series S does not have a disc drive.

You may need to purchase the digital version in order for you to play the game. If you just recently purchased the game, you can also check on the store where you purchased it if you can get a replacement wherein you'll ask for a digital code instead of the physical disc game.

I’m confused how will it work for both if one is physical and the other is digital

Both consoles are digital...it's just the Series S is Digital only whereas the Series X is both Digital and Physical.

75th Anniversary Edition Pre-Order can be downloaded to the Series S, the Series X and the Xbox One

You have names mixed up. There are no consoles named the Xbox One Series S or Xbox One Series X.

The Xbox One/One S/One X all have a disc drive.

The Xbox One S All Digital Edition does not have a disc drive.

The Xbox Series X has a disc drive.

The Xbox Series S does not have a disc drive.

All the consoles can play backward compatible games. The Series S and One S all digital edition can only play games digitally downloaded.

The other consoles can play games from disc and from digitally downloaded,

I hope this clears things up for you.