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 Z7 II has a
46.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Dual Expeed 6 processor.
Another similarity between these two cameras is that both Nikon Z7 II and Sigma Quattro H sensors lack anti-alias (Low-Pass) filters. 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 Z7 Mark II sensor size comparison.
As seen above, Nikon Z7 II 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.