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, Sony A6500 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor. Sigma Quattro H's sensor provides 21MP more than Sony A6500'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 Quattro H and A6500 sensor size comparison.
As seen above, Sigma Quattro H has a 1.3x Larger sensor area than Sony A6500. 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.