You can fill the monitor but if your device output resolution is different from what the monitor can support on a per pixel basis it will either be squished, bits chopped off or re-scaled to fit in some way (maybe black bars at each end depending on the resolution used). Unless your device outputs the exact resoultion of the monitor you are losing image (the top/bottom are squished or not visible) or stretching to fit (upscaling) one way or the other. The only real way to be 100% sure how UW will work with the PS5 is actually plugging one in and seeing what video modes are available once it has detected the display, i personally think it will be fixed to max 4k (or will maybe do 3440*1440 if you are lucky) for performance reasons. Sadly i expect an image in the centre of the monitor and black bars either side from my 5120*1440, even with a lower resolution UW the image will not be 1-1 pixel mapped unless the PS5 outputs the precise resolution of the monitor so will work, but will not be as detailed as a true 4k display.