Game clip are pixelated on every game xbox/xbox 360/xbox one and xbox series x games

which is why the new Xbox series X is supposed to be able to play almost every game from every generation of Xbox console (including the original).

That's marketing spiel that's been misinterpreted. From how I understand it, the Xbox Series X will run most if not all Xbox One games. Which means at best, 100% BC with the Xbox One. It will also play all the games that currently on the Xbox One BC list
on the Xbox One.

Nowhere have said it will play almost, or all Xbox games from all the Xbox generations.

The problem with making all old games work is a hardware one. The Xbox 360 used a PowerPC 32-bit processor. The Xbox One and Series X use AMD X64 based processors. The Xbox One and the Series X are like PCs and they share the same processor architecture.

BC is achieved on the Xbox One because the BC team (with zero help from the developer) take the old 360 game and reverse engineer it to run on the Xbox One, taking the original 32-bit PPC executable archive and turning it into a 64-bit X64 executable. There's
no software emulation for the game. The only thing emulated in software is the 360 OS.

To run most games, or indeed, every game would require a software emulator for 360 games. A pure software emulator is a lot of hard work, far to complex to explain here.

microsoft i believe are making every game on xbox one 360 and original all compatable on series x

Xbox series X will be the next console of the Xbox franchise. Like the jump from Xbox 360 to Xbox one. You will be able to play you're Xbox one games on the Series X and the games coming out will be out on Xbox One as Xbox series X for a limited time.
After a while they will quit making games for Xbox one and focus on Xbox Siries X

Most of games are great on Series S, speacially optimisated ones for S|X

But FIFA 22 is a big problem because I bought the Series S speacially for FIFA but the game looks like from Xbox 360....

We need your support, I hope they will fix this awful / pixelated graphics

Many thanks

Based on my understanding of how the Series X is going to work, it should be playable on that system.

Forgive me but your understanding is wrong.

Series X is going to play only the current Xbox 360 and original Xbox games that have been made BC until now, and most, if not all, Xbox One games. It won't play every Xbox game.