Canon 77D has a
24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 7 processor. On the other hand, Canon 20Da has a
8.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor .
Canon 77D's sensor provides 16MP more than Canon 20Da's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 77D and 20Da sensor size comparison.
Canon 77D and Canon 20Da have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.