When Vg is applied, electric field forms in the substrate that attracts electrons towards the gate. At the surface since electrons cannot pass through the oxide they stay there and the quasi fermi level for electrons gets closer to conduction band, or in other words there is band bending. This causes the p type semiconductor to have n type properties. When you remove the voltage it returns back to p type (the inversion stays as long as the voltage is applied). For the second question, PN junction does not behave in the same way because there is no insulator between p side and n side.

Strong inversion means when applied gate voltage positive to the N MOSFET it can have the p substrate it contains majority of holes and minority of electrons holes get attracted at the surface of gate terminal and in the psubstrate region is electrons both the concentration is equal it is strong inversion and when the concentration of is not equal that is said to be week inversion

in weak inversion concentration of electron in the boundary is less than concentration of holes in bulk but in strong inversion concentration of electron in the boundary is greater than concentration of holes in the bulk(for nMOS)

fermi level of semiconductor is connected to ground and as oxide act as insulator so there will be no electric field in semiconductor therefore fermi level is constant