l4d for xbox series s & x, used to play this game alot

Xbox One games can be played on the Series S/X, but Series S/X games cannot be played on the One.

Both games use smart delivery, so can only download the Xbox Series X|S version of the game on the Xbox Series X|S. If you want to play the Xbox One versions, then playing on the Xbox One is your only option.

Because on my Xbox series x, it only shows 'Optimized for series x/s' but if a xbox series x version of a game is NOT 'optimized' how would i know if I'm playing a series x/s version of a game?

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.

In your situation, I would go with the Xbox Series S,

The Series S is aimed at 1080p / 1440p gaming,

The Series X is aimed at 4K gaming,

Getting a Series X would mean that your spending an extra £/$200 to not play at 4K,

You could use that £/$200 saved by getting the Series S and a 1Tb SSD expansion card for about the same price as the Series X,

https://youtu.be/VIHny-XP8jM