how do you exit a game when playing on the xbox series s?

Press the Xbox button (the large X), not hold it down, and it will take you back to the dashboard. From there, if you highlight the game and press the menu button on the control (the 3 lines), you'll have the option to Quit the game.

When you mean you exit and it crashes, do you mean that you're exiting out of the Scratch & Match back to the S&S+ main menu, or do you mean you're exiting S&S+ back to the dashboard?

when you said series did you mean Xbox series type and

if Xbox series s/x has cross play option for mw2 it's it possible from to download mw2 on Xbox series s if

it has cross play on this game.

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 have several disc games for my Xbox One, i am buying a Series S, but how will i pay them on it?

You wouldnt. Xbox Series S can only play digital games so if you want to play them on Series S you would have to repurchase them digitally.