Both Nikon Z7 and Nikon D850 have 46.0 MP resolution sensors but Nikon Z7's sensor is Full frame (35.9 x 23.9 mm ) and Nikon D850's sensor is Full frame (35.9 x 23.9 mm ).
Since Nikon D850'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 Z7.
One other difference between these two cameras that is worth mentioning is that Nikon D850'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 moire occurring in certain scenes.
Below you can see the Z7 and D850 sensor size comparison.
Nikon Z7 and Nikon D850 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Nikon Z7 and Nikon D850 sensors have been tested by DxO and the results show that D850 has a better overall score of 100, 1 points higher compared to Z7's score of 99.