Fujifilm X-T3 has a
26.0MP APS-C (23.5 x 15.6 mm ) sized X-Trans CMOS 4 sensor and features X-Processor 4 processor. On the other hand, Sony A6300 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features BIONZ X processor.
Another difference between these two cameras is that Fujifilm X-T3'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 X-T3 and A6300 sensor size comparison.
Fujifilm X-T3 and Sony A6300 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 Sony A6300 has 8% larger pixel area (15.28µm2 vs 14.12µm2) compared to Fujifilm X-T3, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.