Both Nikon D3400 and Canon T7 have 24.0 MP resolution sensors but Nikon D3400's sensor is APS-C (23.5 x 15.6 mm ) and Canon T7's sensor is APS-C (22.3 x 14.9 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 T7.
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 T7 sensor size comparison.
As seen above, Nikon D3400 has a 1.1x Larger sensor area than Canon T7. 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.