Canon SL3 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor. On the other hand, Canon 1200D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor.
Canon SL3's sensor provides 6MP more than Canon 1200D's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the SL3 and 1200D sensor size comparison.
Canon SL3 and Canon 1200D 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 Canon 1200D has 33% larger pixel area (18.54µm2 vs 13.84µm2) compared to Canon SL3, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.