All things are made of matter. Matter has atoms (and other fundamental particles) as it’s building block.
These fundamental particles obey electromagnetic and probabilistic laws.
Quantum physics is a theory that makes the predictions about the statistics of these microscopic objects often with implications for bulk phenomena.
Statistical physics describes what happens when the number of these microscopic objects become very large, leading to the bulk phenomena.
Given the irreducibility of matter beyond the fundamental particles (per the definition of ab initio above), and since the microscopic objects obey laws of these models, Quantum physics and Statistical physics are ab initio models.
I think classical electromagnetism is also an an ab initio model since these microscopic objects obey it’s laws.
The first principles should be the Coulomb’s law.