Samsung NX500 has a 28.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features DRIMe 5 processor. On the other hand, Fujifilm X-A3 has a 24.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features EXR Processor II processor.
Another difference between these two cameras is that Samsung NX500'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 NX500 and X-A3 sensor size comparison.
Samsung NX500 and Fujifilm X-A3 have 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 Fujifilm X-A3 has 16% larger pixel area (15.37µm2 vs 13.18µm2) compared to Samsung NX500, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.