Fujifilm X-T100 has a
24.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor . 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 moiré occurring in certain scenes.
Below you can see the X-T100 and Z fc sensor size comparison.
Fujifilm X-T100 and Nikon Z fc 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. On the other hand, since Nikon Z fc has 16% larger pixel area (17.85µm2 vs 15.37µm2) compared to Fujifilm X-T100, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.