Buying a game on Series X and playing it on Xbox One

All Xbox One games can be played on the Series S/X via backwards compatibility. Kind of like how you can play 360 games on the One now. You would be playing the One version of the games.

With Cross Gen Editions/Smart Delivery, you're buying the One version and get an upgrade to the Series S/X version of the game when you get the new console instead of having to buy the Series S/X version and you an take advantage of some of the upgraded
features.

I just bought a brand new Xbox One X, but when I go to the store, Flight Simulator and other Xbox Series X/S games say they are "not playable on this device" and I can't get Flight Simulator to install on my brand new Xbox One X because it says it's "not compatible". Very frustrating.

So you bought an Xbox One X and expected to be able to play an Xbox Series X game on it?

Why would a game made for an Xbox Series X work on an Xbox One X?

If you want to play Xbox Series X/S games I would take your brand new Xbox One X back to the store you bought it and see if they will take it back and you could try and buy an Xbox Series S (Series X consoles are very hard to find, but a Series S can be found fairly easilly)

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)

Yes, if you purchase the game from the Xbox 360 marketplace, you can play it on the Xbox 360, Xbox One or Xbox Series X|S.

As for what Hardcore Games said, you can still buy Xbox games for the Xbox 360, and play them on the Xbox 360, and if they are backward compatible, play them on the Xbox One & Xbox Series X|S.

Microsoft introduces "Smart Delivery" with the Xbox Series X, which means, that if you own an existing game (like Minecraft in your example, or Gears 5) or a new game comes out for Xbox One and Xbox Series X with improved graphics on Series X, you won't
have to buy the game twice.

With Smart Delivery you will play the "enhanced" version on Series X, and the "normal" version on Xbox One.

For more info about Smart Delivery and other terms used with the Series X check this page: https://news.xbox.com/en-us/2020/03/16/xbox-series-x-glossary/