Am physicist. Quantum particles are observable. Often things are observable because you can observe their effects. Can't measure a top quark, but you can measure the electrons and photons when it decays. And their energy and how often it happens lines up with theories developed to describe some different thing.

Theory of everything can't exist, be abuse Gödels incompleteness theorem. But no physicist doubts that all the microscopic stuff gives rise to the macroscopic.

The beginning of the universe you can see in the microwave background or something. So again that's just experimentally motivated theory.

Some of us come up with random theories because it's fun. But most of the time the theory is aimed at explaining some thing that we observe and will coincidentally make predictions about other stuff which we can test.

Finding out how dinosaurs sounded like is dope af from a physicists perspective.