Measuring MOSFET input capacitance?
You are measuring the lump capacitance in accord with this standard model,



You even have observed the Miller Plateau.

The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.

Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.