In internet resources, only $p_z$ orbitals are shown to overlap with $s$ orbitals to form sigma bond. But my teacher says that according to Valence Bond Theory, $s$ orbital overlaps with all three $p_x$, $p_y$ and $p_z$ orbitals to form sigma bonds. I am really confused about this. I heard that the concept changes when it comes to molecular orbital theory. Only similar orbitals (like $1s$-$1s$,$2p_x$-$2p_x$,$3p_y$-$3p_y$) form bonds but ($2s$-$2p_z$) do not.
Please clarify if there is any difference in concepts of overlapping between 'Molecular orbital theory' and 'Valence Bond Theory'.
