The Xbox app has lots of PC games. I use a 4K panel so I have some expectation as this panel is not exactly the latest and it’s actually my second 4K panel.
The image shows clearly that the images are large enough but they do not span across the full width of the panel. This makes the store as bad as Steam which seems to be designed for 800×600.
Obviously Xbox does not have an extensive selection of games available. The again they sell all their games on Steam which is the dominant store for PC gamers.
The site uses an adaptive theme so it does not matter what kind of device visits. The site is at its best at 2560×1440 or above.