Sigma fp has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor . On the other hand, Nikon Z9 has a 46.0MP Full frame (35.9 x 23.9 mm ) sized Stacked CMOS sensor and features Expeed 7 processor.
Nikon Z9's sensor provides 21MP more than Sigma fp'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 fp and Z9 sensor size comparison.
Sensor Size and Resolution Comparison image of Sigma fp and Nikon Z9 Cameras
Sigma fp and Nikon Z9 have the same 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 Sigma fp has 89% larger pixel area compared to Nikon Z9, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.