Sony A6100 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Nikon Z fc has a 21.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features Expeed 6 processor.
One other difference between these two cameras that is worth mentioning is that Nikon Z fc's sensor doesn't have an 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 A6100 and Z fc sensor size comparison.
Sony A6100 and Nikon Z fc have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.