I don't know whether the proper term is Physicalism or Physical Reductionism (it's Reductive Physicalism--thank you, Conifold), but what I'm referring to is the position that everything reduces to physics. My question is, for proponents of this doctrine (assuming there actually are any), which branch of physics does everything reduce to? Quantum Mechanics? Particle Physics?
The reason I ask is that not even all branches of physics reduce to one branch of physics. With a few exceptions, different physical theories interlock and agree with one another in a pleasing way that reassures the student that it isn't all just BS. For example, Newtonian mechanics can be seen as an approximation to relativistic mechanics for smallish amounts of energy. This is what you'd hope for, since a theory that gave different results from Newtonian mechanics in everyday circumstances isn't likely to be right. And it is also true that statistical mechanics, both the classical and quantum versions, give some insight into the results of thermodynamics by going down a level--the microscopic level of molecules.
However, it is not the case that thermodynamics reduces to statistical mechanics. It is its own separate discipline based on generalizations from macroscopic empirical observations, and its validity does not depend on a particular microscopic statistical interpretation. Even if some flaw is discovered in the statistical approach, it does not mean that the laws and results of thermodynamics are suddenly in doubt. The second law of thermodynamics doesn't go away just because nobody has figured out how to derive it as a theorem in statistical mechanics (That's probably not the most relevant example, since a lot of people are apparently satisfied that it has been, and it's not central to my point). Also, as Conifold pointed out in the comments below, there is still a lack of conceptual reduction of thermodynamics to statistical mechanics. Thermodynamic concepts have to be imposed on the microscopic picture, rather than falling out naturally.
So when you look at what physical theories are actually like, you see that they are a bit more like a quilt than a perfectly nested set of Russian dolls. Thus I'd like to know a bit more specifically what people have in mind when they say everything reduces to physics.