Both Nikon Z30 and Nikon Z50 have APS-C sized 21.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Nikon Z50 and Nikon Z30 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 moiré occurring in certain scenes.
Below you can see the Z30 and Z50 sensor size comparison.
Nikon Z30 and Nikon Z50 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.