Quantum Physics is an ab initio model because it uses the first or the simplest of principles that is the basic Schrodinger equation and builds up the whole physical world. All the physical phenomena around us are explained by using just the inputs of Schrodinger equation and few other mathematical equations and physical constants. Nowhere, in the methods any experimental information used.
Similarly, classical mechanics is an ab initio model as well because large physical phenomena are explained via a simple set of equations i.e newton’s equation of motion which cannot be simplified further.
Classical electromagnetism- I am not sure about the answer to this one.
I guess Yes, it is an ab initio model and Maxwell equations are the first principles.