GoPro Hero9 Black has a
23.0MP (1/2. ) sized CMOS sensor and features GP1 processor. On the other hand, Olympus TG-850 iHS has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VII processor.
GoPro Hero9 Black's sensor provides 7MP more than Olympus TG-850 iHS'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 Hero9 Black and TG-850 iHS sensor size comparison.
As seen above, Olympus TG-850 iHS has a infx Larger sensor area than GoPro Hero9 Black. 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.