Is it possible to play with the XBOX one S UHD?
Yes, but only upscaled 4k, but basically it works, yes!
Yes, but as already described, this is only scalable.
This means that the console adds the missing pixels itself in the case of a Full HD, ie 1080 pixels, on a UHD TV. To do this, neighboring pixels are copied.
You can only get real native 4k with the One X. There the game specifies the pixels. If supported, each pixel is its own and not a copied one.
Imagine the difference. If you play a DVD on a Blu-ray player and then the same film as Blu-ray. Depending on the quality of the upscaling, the difference is enormous but always visible.