Leica SL2 has a
47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro III processor. On the other hand, Nikon Z6 II has a
25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Dual Expeed 6 processor.
Leica SL2's sensor provides 22MP more than Nikon Z6 II'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 difference between these two cameras is that Leica SL2's sensor lacks anti-alias (Low-Pass) filter. 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 SL2 and Z6 Mark II sensor size comparison.
Leica SL2 and Nikon Z6 II 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.