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)?”

Answer:
Quantum physics is an ab initio model because it doesn’t use experimental observations to build the theory. It is built from scratch, the first principle.
classical electromagnetism is not an ab initio model because it is developed based on experimental observations. The principles of classical electromagnetism are all experimental findings.