Sony A7R III has a 42.0MP Full frame
(35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, 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.
Another similarity between these two cameras is that both Sigma Quattro H and Sony A7R III 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 A7R III and Quattro H sensor size comparison.
As seen above, Sony A7R III 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.