Why bond polarity and molecular geometry determine molecular polarity?
Because molecular polarity results from the VECTOR sum of the individual bond dipoles. Vectors have magnitude and direction, so polarity is in part a function of geometry.
Take carbon tetrafluoride,
The same reasoning applies to
So how to conclude? Two factors determine molecular polarity: (i) bond polarity (which can be assessed by the electronegativity difference between the bound atoms); and (ii) the VECTOR sum of the bond dipoles.