1. Why do we call quantum physics an ab initio model?

Ans- Ab initio literally means, from the beginning. Ab initio methods are methods which take no assumptions or empirical model but only use physical constants. So since quantum physics is an ab initio model.

2. Why do we call classical mechanics an ab initio model?
Ans- For the same reason mentioned above, classical mechanics is an ab initio model. It also takes no assumptions or empirical models but only uses physical constants.

3. Is classical electromagnetism is an ab initio model (and if it is one, what are its first principles)?”

Ans- Classical electromagnetism is generally not considered as an ab initio model, because it uses some underlying assumptions.