1. Why do we call quantum physics an ab initio model?
Physicists have taken observations from several landmark experiments (e.g. Young’s double slit, Stern-Gerlach etc.) and come up with a mathematical model to describe what is observed in nature.
2. Why do we call classical mechanics an ab initio model?
The exact same argument as presented above except with different experiments.
3. Yes electromagnetism is an ab initio model as well. First principles include 1) charge is conserved; 2) opposite charges attract, same charges repel.