Both Canon SL3 and Nikon D3400 have 24.0 MP resolution sensors but Canon SL3's sensor is APS-C (22.3 x 14.9 mm ) and Nikon D3400's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D3400'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 Canon SL3.
One other difference between these two cameras that is worth mentioning is that Nikon D3400's sensor doesn't have an 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 SL3 and D3400 sensor size comparison.
As seen above, Nikon D3400 has a 1.1x Larger sensor area than Canon SL3. 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.