Both Leica SL and Nikon D3500 have 24.0 MP resolution sensors but Leica SL's sensor is Full frame (36 x 24 mm ) and Nikon D3500's sensor is APS-C (23.5 x 15.6 mm ).
Since Leica SL's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D3500.
Another similarity between these two cameras is that both Nikon D3500 and Leica SL 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 SL and D3500 sensor size comparison.
As seen above, Leica SL has a 2.4x Larger sensor area than Nikon D3500. 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.