Can Xbox One S/X users purchase the Expanded and Enhanced version of GTA5?

No as the they aren't series consoles

The Xbox One S is a variant of the Xbox One console. Xbox Series X/S is the newer generation of consoles. Xbox Series X/S enhanced games are only for the newest consoles not the Xbox One S. You would need to purchase the Xbox One version of the games not the X/S version of them.

HI,

This an Xbox One X enhanced game. But to get the enhancement you'll need either a Xbox One X or either one of the newer Series X|S machines. The Series S is a digital only console though.

On an original Xbox One or One S, it will be the BC 360 game with no enhanced graphics.

You'll get access to an enhanced version regardless of disc or digital, but you need the right console to be able play the enhanced version.

This is an incorrect assumption. You can insert an xbox one disc and download updates that may improve visuals and frame rates on Xbox Series X/S but in order to play you still need the disc unless you own the digital version of it.

Xbox Series X will utilize the Xbox One X version of enhancements while Xbox series S will use the Xbox One S enhancements.

You will not get a free game by borrowing or renting a physical version of a game and installing it on Xbox Series X/S.

you can use the version of the Xbox One X

It's not that simple. The version of SEIKO for the Xbox One X can utilise the extra system RAM (Random Access Memory) available to that model of Xbox. While the Series S has significantly more processing power than both the Xbox One X and the original Xbox One, it only has around the same amount of system RAM as the original Xbox One.

This is the reason that none of the Xbox One X Enhanced games are enhanced on the Xbox Series S via backwards compatibility. They have to run the original Xbox One code via BC as Series S lacks RAM.