Quantum physics is an ab initio model because we start with the Schrödinger Equation and from there we can find all properties (position, momentum, etc.) of the given situation.
Classical mechanics is also an ab initio model because the only thing we need are Newton’s equations and conservation of energy and momentum. From there we can simulate all sorts of things ranging from planets orbiting a sun to biljard balls bouncing around on a table.
I think that classical electromagnetism is also an ab initio model. There you start with the rules that two oposite charges attrackt (almost like gravity) and the same charge repels. From this we get moving charges and those generate magnetic fields.