Does Xbox automatically switch resolution and refresh rate?

Exactly. Just make sure your TV really supports 120hz.

I have a TCL 6-series 55” 4K TV (2020 model).

Hi,

Could you provide us with the make a model of your TV?

If your TV is only HDMI 2.0 then it won't actually natively support 120Hz. Only certain supported HDMI 2.0 monitors can run 120Hz, but are generally limited to 1440P resolution.

It's more likely your TV has a 120Hz mode purely for Motion interpolation. This can make lower frame rate video from TV and movies appear smoother. However, with games Motion interpolation will add a huge amount of display lag. Real 120Hz TV's are newer HDMI 2.1 displays.

Xbox games are always rendered at the resolution they were designed to run at. So a 4K game On a 1440P display will can run internally at 4K and is then automatically scaled to whatever resolution the display is set to.

Hello, SacredChaosKing. Thanks for reaching out. I'm Lucas, an Independent Advisor and a Microsoft user just like you. I'll be glad to help you out.

I believe your console will run at 1440p 120hz if the game and the TV supports it. If the game or the TV does not support it, the game will then try to be run at 4k 60hz. But it will always prefer 1440p 120hz since it is the display setting you chose on the console.

Feel free to ask back any questions.

I have a TCL 6-series 55” 4K TV (2020 model).

This one? https://www.rtings.com/tv/reviews/tcl/6-series-r635-2020-qled

It's not 4K 120Hz, but it does support 1440P or 1080P at 120Hz and it's one of the few TV models that actually supports VRR with HDMI 2.0.