The question:
Why do we call quantum physics an ab initio model? Why do we call classical mechanics an ab initio model? Is classical electromagnetism an an ab initio model (and if it is one, what are its first principles)?

The response:
We call quantum physics an ab initio model because it starts from the fundamental particle based equations to explain the nature of and behavior of such particles at relatively high speeds equivalent to speed of light that cannot be explained in classical way, so these were explained in new methods from the wave nature of particles, thus from the fundamental principles.

Classical mechanics is an ab initio model because it depends on the fundamental laws of Newton of Laws.

Classical electromagnetism is an an ab initio model because its based on Maxwell’s equations. which are again fundamental within the subject of electromagnetism.