The basic mechanisms are listed below.

a) Electronic Polarization

b) Ionic Polarization

c) Space charge Polarization

d) Orientational polarization.

Total polarization is actually sum of (a) , (b) and (d).Generally, the contribution of space charge polarization is very small as compared to other three.

I second that,

https://eng.libretexts.org/Core/Materials_Science/Optical_Properties/Dielectric_Polarization

although mostly ionic in most dielectrics.

There are two types of dielectrics, polar and non-polar. Non-polar dielectrics do not have a permanent electric dipole moment, when you apply a field, you generate dipolar moments in the direction of the applied field. You can see the microscopic polarization (the response to the field), as the sum of all the dipolar moments induced. For polar dielectrics, when you applied a field, the pre-existent dipolar moments suffer a torque and tend to align with the applied field, in this case the polarization can now be seen as an average of all the dipoles orientations.