Both Nikon D850 and Nikon Z 7 have 46.0 MP resolution sensors but Nikon D850's sensor is Full frame (35.9 x 23.9 mm ) and Nikon Z 7's sensor is Full frame (35.9 x 23.9 mm ).
Since Nikon Z 7'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 D850.
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 D850 and Z 7 sensor size comparison.
Nikon D850 and Nikon Z 7 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.