Why do we call quantum physics an ab initio model? Why do we call classical mechanics an ab initio model?
Quantum Physics is an ab initio model as it makes predictions based on mathematical model which in this case is Schrodinger Equation. Similarly in Classical Mechanics, Newton’s equations are the mathematical model.
Is classical electromagnetism an ab initio model (and if it is one, what are its first principles)?
Yes, classical electromagnetism is an ab initio model based on Maxwell’s Theory on Electromagnetism.