Depends on the particular FF and APS-C cameras. MOST FF sensors have bigger pixels (larger pixel pitch) than APS-C cameras. Noise tends to increase as pixel si

Depends on the particular FF and APS-C cameras. MOST FF sensors have bigger pixels (larger pixel pitch) than APS-C cameras. Noise tends to increase as pixel size decreases. If you crop a FF image down to APS-C size, it'll end up with lower resolution. So the narrower angle of view that an APS-C gets you gives an apparent zoom that can't be achieved with such a FF sensor. Whereas a FF sensor can take wide-angle shots better and has lower sensor noise. There are exceptions to this. Notably the Sony a7R III, a7R IV, a1, and the Cannon R5 are FF cameras with small enough pixel pitches that they're actually higher resolution after cropping to APS-C than most APS-C cameras. So in these cases, there's no difference between shooting with the FF camera in APS-C mode and an actual APS-C camera.