Sony A6400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X 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 22MP more than Sony A6400'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.
Below you can see the A6400 and Z7 sensor size comparison.
As seen above, Nikon Z7 has a 2.3x Larger sensor area than Sony A6400. 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.
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. Sony A6400 and Nikon Z7 sensors have been tested by DxO and the results show that Z7 has a better overall score of 99, 16 points higher compared to A6400's score of 83.