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, Sony a3500 has a
20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features BIONZ processor.
Below you can see the A6100 and a3500 sensor size comparison.
Sony A6100 and Sony a3500 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 a3500 has 21% larger pixel area (18.50µm2 vs 15.28µm2) compared to Sony A6100, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.