Yes the framerate and display refresh rate doesn’t need to match.So it is imposible to set 120hz display mode on 60fps game? Series X in question.
That is why VRR like solutions were created.
A 30fps game can be displayed in a 120Hz display... in fact each frame will be displayed 4 times in the display... solid 30 or 60fps games is displayed very well on 120Hz display.
The issues starts with non-multiple framerates like 45 or 50... or framerates that keep changing so you need VSync alternative... VRR... etc to make it proper work.