Both Nikon D3400 and Leica M Edition 60 have 24.0 MP resolution sensors but Nikon D3400's sensor is APS-C (23.5 x 15.6 mm ) and Leica M Edition 60's sensor is Full frame (36 x 24 mm ).
Since Leica M Edition 60'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 D3400.
Another difference between these two cameras is that Nikon D3400'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 moire occurring in certain scenes.
Below you can see the D3400 and M Edition 60 sensor size comparison.
As seen above, Leica M Edition 60 has a 2.4x Larger sensor area than Nikon D3400. 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.