Both Nikon Z 7 and Nikon D850 have 46.0 MP resolution sensors but Nikon Z 7'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 Z 7.
Another difference between these two cameras is that Nikon D850'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 Z 7 and D850 sensor size comparison.
Nikon Z 7 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.