Sigma fp L has a
61.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor . On the other hand, Sony A7S III has a
12.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz XR processor.
Sigma fp L's sensor provides 49MP more than Sony A7S III'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 L and A7S III sensor size comparison.
Sigma fp L and Sony A7S III have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.