This is what you call screen centering.
I'm no aim god, but nonetheless, this is pretty interesting.
Screen centering is basically always looking at the center of your screen. So even without a crosshair, you would roughly be able to tell where the exact center of your screen is. This is why it is advised to look at the target you're aiming at instead of the crosshair, if your screen centering is good enough, your crosshair will already be aligned with the target you're looking at. Its a good practice to keep your eyes on the center of the screen while using your mouse to look around, somewhat like you're controlling a CCTV where only the center is in focus.