I have a Full HD TV and my question in what graphics settings can I play on the xbox one s (e.g. 1080p 60fps)?

For example in the game Forza Horizon 4

It depends on the game, how much FPS you have. Full HD can't play 4k.

Full HD is 1920x1080p. The fps depend on the game, but you have a maximum of 60hz.

So I set it to 720p on my one s because it was read everywhere that the one can only represent 900p anyway. And I (was the opinion) through 720p that I have somewhat better performance.