I bought a digital game on xbox one, if i buy the series x will i be able to play the game...

If you bought the Xbox One version of the game or the Digital Deluxe version of the game, you will be able to play the game on your One X when the One version is released in April. If you only bought the Series S/X version, it will not work on the One X.

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)

Games are tied to your account, not your console. Your digital games come with you when you upgrade.

Just need to sign in with same account on the Series X.

Yes, Xbox One game discs should work on the Xbox Series X consoles, as they are backwards compatible. So if you buy the physical copy of Hogwarts Legacy on the Xbox One, you should be able to play it on the Xbox Series X consoles, However If you don't have a Digital Copy or Disk Reader technology that reads disks and install them on you're Console it'll will not work on Series S as it's all Digital.