why is my xbox one s outputting bad graphics

...example forza horizon 5. it used to run at high graphics 1080p 60fps, but now its 30-ish fps with 720p cars and grass that looks like 420p...

Not sure where you've got the idea that FH5 runs at 60fps on the Xbox One. None of the Forza Horizon games have run at 60fps on the Xbox One. Forza Horizon 5 certainly doesn't run at 60fps.

https://www.eurogamer.net/digitalfoundry-2021-forza-horizon-5-xbox-platform-comparison

"The thing is, it still works. In fact, most of my initial play-testing was on the base Xbox One, simply because I wanted to see how Playground did it. Because it's content complete, because the key features are all included and because the result has been tailored by Playground to mask as many of the limitations as possible, it's still a worthy sequel. It also helps that the developer has retained 1080p resolution and 4x MSAA anti-aliasing: Forza titles have always looked pristine in this regard and FH5 is no exception. Dynamic resolution scaling to 810p is there (DRS being a first for Forza titles as far as I'm aware) but it's a tool of last resort as opposed to a core element of the visual make-up. In fact, Playground deploys what it calls 'DRS Plus', dropping back fidelity on cube-map reflections and shadow updates before lowering resolution. That leads us onto the other aspect of FH5 on Xbox One that emphasises quality - the 30fps target frame-rate barely budges."

Check your display in your settings

Unfortunately you went from a PS4 to an Xbox One S, and the base PS4 is more powerful than the Xbox One S. This means that the game may be running at a lower resolution etc.

Ok thanks, i guess its better to wait for the Series X to come out then!
The Xbox Series X is a 4K gaming console, yes you can use it at 1080p but your not going to get or see the full potential out of its performance unless you have a 4K TV or Monitor that has HDMI 2.1, in my opinion, if your not getting the full potential out
of it , it's a waste of money,

I would wait and see if Microsoft reveals the Xbox Series S as that will be more aimed at 1080p / 1440p and will cost much less than the Xbox Series X.

1-check xbox display resolution at settings, display, and also check tv resolution too. then restart chosen game for changes to go to effect.

Unfortunately you went from a PS4 to an Xbox One S, and the base PS4 is more powerful than the Xbox One S. This means that the game may be running at a lower resolution etc.