Sigma Quattro H has a
45.0MP APS-H (26.6 x 17.9 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. 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.
Another difference between these two cameras is that Sigma Quattro H'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 moiré occurring in certain scenes.
Below you can see the Quattro H and Z9 sensor size comparison.
As seen above, Nikon Z9 has a 1.8x Larger sensor area than Sigma Quattro H. 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.