Chest voice is stretching/curdling the vocal folds from their bottom (the curdling makes for the high overtone content of chest voice and couples vibration better into, well, the chest), falsetto is stretching them by pulling at their fixtures. Because of the mechanics of the larynx, the involved muscles and operations and configuration of the vocal folds are not quite independent, so employing them both at the same time is a comparatively delicate operation.
If you use mainly one or the other, because of the considerably different configuration of the vocal folds and the mechanism for tensing them, you are almost dealing with two different instruments. How much their pitch ranges overlap is basically an anatomical detail. And how much may be accessible to the use of more than one of those mechanisms to what degree, is to some degree anatomical, but to a high degree also depending on practice dealing with what amounts to an unstable configuration with a tendency to flip to one or the other mode of operating exclusively.