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, Leica SL has a 24.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor. Sony A7R III's sensor provides 18MP more than Leica SL'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.
Another similarity between these two cameras is that both Leica SL 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 moire occurring in certain scenes.
Below you can see the A7R III and SL sensor size comparison.
Sony A7R III and Leica SL have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.