Nikon D3500 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Canon 1D X III has a
20.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic X processor.
Another difference between these two cameras is that Nikon D3500'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 D3500 and 1D X MIII sensor size comparison.
As seen above, Canon 1D X III 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.