Nikon Z30 has a
21.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features Expeed 6 processor. On the other hand, Nikon Z6 has a
25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor.
Another difference between these two cameras is that Nikon Z30'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 moiré occurring in certain scenes.
Below you can see the Z30 and Z6 sensor size comparison.
As seen above, Nikon Z6 has a 2.3x Larger sensor area than Nikon Z30. 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.