Both Nikon Z5 and Nikon D7100 have 24.0 MP resolution sensors but Nikon Z5's sensor is Full frame (35.9 x 23.9 mm ) and Nikon D7100's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon Z5'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 D7100.
One other difference between these two cameras that is worth mentioning is that Nikon D7100's sensor doesn't have an 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 moiré occurring in certain scenes.
Below you can see the Z5 and D7100 sensor size comparison.
As seen above, Nikon Z5 has a 2.3x Larger sensor area than Nikon D7100. 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.