xbox series s cant display 1440p@120hz on monitor that can display 1440p@180hz

I think that might be the root of the problem according to the requirements on the Xbox support page.

because the chips in the HDMI part or what determine how fast it goes. The only reason why you need an HDMI 2.1 cable is because it has extra wires in it for extra channels

180 Hz is not a VeSAstandard timing, an Xbox does not support it

Monitors are TVs made as cheaply as possible that rely on the graphics card to make up for the fact that they included the most basic hardware possible. SOC are very expensive on the bill of materials, and by leaving out expensive scaler and image processing and turning it over to the PC

When it comes to displays, you definitely get what you pay for. The faster processors and things like that will result in far superior motion image quality. Try playing a game like rock band where the lyrics scroll horizontally on a cheap display and you'll see the difference

but i don't understand why this work for 1080p then?

am using an HDMI 2.1 cable but it turns out i have HDMI 2.0 ports

the monitor must have exact 120hz listed in its edid for the xbox to work with it, pc's can use any refresh rate and are not fixed, but a lot of gaming monitors have 119.3hz listed or roughly that, so if 120hz is not in the edid then you cannot use 120hz on that monitor.