Aspect ratio is correct but it won't fit to screen when using mobile preview app
when I have a 360x640 artboard it has the exact same aspect ratio as a 1080x1920 phone. however when viewing the proto on that phone, or any of several phones that match that aspect ratio, the artboard doesn't fit to the screen, and instead all the way around the board it leaves a black space.
Surya Aditya Paturi commented
XD prototypes never upscale the artboards. That means the artboards does not stretch beyond their original size. However, they might get downscaled if the screen size is less than artboard size.
One more thing to note is although the screen physical resolution is 1080x1920 for all practical purposes it is much less than that by factor called scaling factor. So, if you assume the scaling factor is 2.5 then your 1080x1920 will be treated as 432x768 which is still more than your artboard size 360x640 and hence the black spaces around the artboard. If you want your artboard to fill the entire screen you might have to increase the artboard dimensions.