Ab initio roughly translates to First Principles

For things that move much slower than photons and are much bigger than electrons, newton’s laws hold, infact every behaviour of such object can be traced back to the laws, hence classical mechanics is the ab initio
When objects move and do their thing, much smaller scale of size say that of roughly atoms or electrons, schrodinger’s equation, the quantum theory is the ab initio
Similarly a particular property of matter known as charge, produces physical phenomena, which are better understood by the conception of fields, when a charged object appears in space it creates electric field, when a charged object moves it creates a magnetic field. In this case the first principle is the Maxwell’s Equations – and so yes electromagnetic theory is ab initio.

In summary anything which cannot be derived from an existing body of knowledge is first principle!