Both Nikon D500 and Nikon D5 have 21.0 MP resolution sensors but Nikon D500's sensor is APS-C (23.5 x 15.7 mm ) and Nikon D5's sensor is Full frame (35.8 x 23.9 mm ).
Since Nikon D5's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D500.
Another difference between these two cameras is that Nikon D500's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D500 and D5 sensor size comparison.
As seen above, Nikon D5 has a 2.3x Larger sensor area than Nikon D500. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.