Canon M6 MII has a 33.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor. On the other hand, Sony A6400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor.
Canon M6 MII's sensor provides 9MP more than Sony A6400'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 M6 II and A6400 sensor size comparison.
As seen above, Sony A6400 has a 1.1x Larger sensor area than Canon M6 MII. 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.