Why do some Xbox 360 games run and look terrible on the Xbox series x even though they were...

Anything changed since you last played, like a new 4K TV? Xbox 360 games were 720P, they won't appear sharp on a 4K TV.

Did you use the fps booster ? If so turn it off along with hdr

I was wondering if all xbox 360 games will ever be compatible with the xbox series x.

The simple answer to that question is no. The reason is a technical one.

Unlike Xbox One games running on Series X, the Xbox 360 used different hardware. Because the hardware was so different the BC team had to convert and repackage Xbox the Xbox 360 games that were added to the BC list, they can't be run natively from their
original discs or run the original digital downloaded versions like Xbox One games can.

Converting Xbox 360 games to run on Xbox One or Series X/S hardware takes a considerable amount of time and each game had to be tested. This is main reason why there is only a limited selection of Xbox 360 games available for the Xbox One and Series X/S
consoles.

The BC program for Xbox 360 and original Xbox One has been suspended. There have been no announcements from Microsoft of if or when it will be restarted. It may never be restarted.

Even if it does, the other problem is licenses. Most sports games including car racing have extensive licensing agreements with third parties. In the case of the NFS games these licenses will be for the cars in the game and any licensed in-game music. Much
of which will have expired.

Original xbox games that play on the 360 don't run very well at all which is why they stopped adding them.They weren't upscaled or optimised at all so look & play pretty terrible on the 360 & on a HD TV.

The original xbox games that play on the xbox one are upscaled & optimised to run a lot better.

It also depends on the game,you have to remember that a lot of original xbox games were only in 480p which looks great on a CRT TV but not so good on a HD TV.

....they should allow us as consumers to have access to the set of code they used for the emulator...

Xbox 360 and original Xbox games technically aren't being emulated. The games were reverse engineered to run on the Xbox One. They are, for all intents and purposes, Xbox One games running inside an Xbox 360/original Xbox virtual machine. They're not 'emulated' in the traditional sense.

This is why when you insert a BC 360 Xbox game, there is an update to download. This is not an update, it is the full game recoded to run on the Xbox One or Series X|S consoles.