Nikon Z6 has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Nikon Z7 has a 46.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor.
Nikon Z7's sensor provides 21MP more than Nikon Z6's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
One other difference between these two cameras that is worth mentioning is that Nikon Z7'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 Z6 and Z7 sensor size comparison.
Nikon Z6 and Nikon Z7 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon Z6 has 86% larger pixel area compared to Nikon Z7, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
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 Z6 and Nikon Z7 sensors have been tested by DxO and the results show that Z7 has a better overall score of 99, 4 points higher compared to Z6's score of 95.