Both Pentax K-1 II and Nikon D810A have 36.0 MP resolution sensors but Pentax K-1 II's sensor is Full frame (35.9 x 24 mm ) and Nikon D810A's sensor is Full frame (35.9 x 24 mm ).
Since Nikon D810A'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 Pentax K-1 II.
Another difference between these two cameras is that Pentax K-1 II's sensor lacks 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 K-1 Mark II and D810A sensor size comparison.
Pentax K-1 II and Nikon D810A have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.