Is there a fundamental ontology in Physics? According to Physics, what are the fundamental "things" (whatever they may be) that in combination define the whole universe, such that they are not themselves defined in terms of other simpler "things"?
If you're thinking of building blocks in terms of something that is located in space and time, and if, when mentioning physics, you're thinking of quantum mechanics, this is an open philosophical question.
There are well defined fundamental types of "particles" in physics (quarks, leptons...) but instances of these types are not really represented as normal particles in a 3D space. They are represented as wave-functions, which are, mathematically speaking, fields in a 3N dimensional space (where N is the number of particles). This is necessary to account for quantum phenomena such as entanglement. But this kind of representation is rather abstract and looks nothing like building blocks of matter in space.
One can go on with this and assume that the purpose of physics is not to represent building blocks in a traditional sense (but perhaps only to predict phenomena or measurements) but some philosophers are dissatisfied and think that we should be able to interpret wave-functions in terms of a distribution of matter in space and time. This has led to a research program known as "primitive ontology". It consists in completing the bare theory with an interpretative layer: pointing at mathematical structures derived from the standard formalism and claiming "this is what really, really exists". Proposals include particle ontology (continuous in time but discrete in space, such as in Bohmian mechanics), matter densities (continuous in space and time) and events aka "flashes" (discrete in both space and time).
An alternative to this program developed by Albert consists in claiming that space is not 3 dimensional but 3N dimensional, so as to read off the ontology straight from the formalism. Our "normal" space would be emergent.
Note that these programs are not pursued by physicists, who tend to have a more pragmatic approach towards physics I would say, but by philosophers seeking a realist picture of the world based on physics. They all have difficulties, only metaphysical arguments favour one or the other (with no consensus), and in any case, contemporary physics is nothing like a straightforward representation of building blocks in space and time like classical physics was: it's a matter of interpretation.
Usually, we consider these seven units, and hence the quantities they represent, to be fundamental: they perfectly satisfy the condition "not defined in terms of other simpler existing things". All other physical quantities can be defined in terms of these units, as far as we know, but there are propositions that the list needs expansion. You can think of these as the building blocks for perception and measurement. They don't just describe composition, but also include phenomenon, which is good because physics discusses both what things fundamentally are, and how they behave.
From a different perspective, I could answer with reference to the Standard model. It features 17 fundamental particles (plus antiparticles for several of them) which make up everything and are organized into very different groups based on their properties like mass and superimposability in spacetime. Briefly, they're 6 quarks, which are (simplification alert) heavy and interact with a particular force (strong nuclear force), leptons (small rest mass, no strong interaction), gauge bosons (force-mediators, sometimes massless), and the Higgs boson.Theories describing a more fundamental composition of the universe exist (most importantly string theory), but are not proven and currently theoretically unprovable, so I'll ignore their existence.
The elements of the universe are substances. Substances are inextricable composites of
- primary matter (materia prima): "that which is in potency to substantial existence"1
- substantial form (forma substantialis): "that which causes substantial existence in act"2.
1. St. Thomas Aquinas, On the Principles of Nature §3
2. idem §5