Both Canon T8i and Nikon D5500 have 24.0 MP resolution sensors but Canon T8i's sensor is APS-C (22.3 x 14.9 mm ) and Nikon D5500's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D5500'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 T8i.
One other difference between these two cameras that is worth mentioning is that Nikon D5500'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 T8i and D5500 sensor size comparison.
As seen above, Nikon D5500 has a 1.1x Larger sensor area than Canon T8i. 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.