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 A9 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-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 moire occurring in certain scenes.
Below you can see the X-T3 and A9 sensor size comparison.
As seen above, Sony A9 has a 2.3x Larger sensor area than Fujifilm X-T3. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.