Sony A7R III has a 42.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Pentax K-1 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor . Sony A7R III's sensor provides 6MP more than Pentax K-1'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.
Another similarity between these two cameras is that both Pentax K-1 and Sony A7R III sensors lack anti-alias (Low-Pass) filters. 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 A7R III and K-1 sensor size comparison.
Sony A7R III and Pentax K-1 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax K-1 has 16% larger pixel area (23.83µm2 vs 20.43µm2) compared to Sony A7R III, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.