"What if the equations only describe what is relevant to the humans"
Which equations? Physics has been on a 'journey of unification', resulting in a structure which fits with a vast amount of observations, as discussed here: Is the idea that "Everything is energy" even coherent? Those observations are still by humans, but we undertake practices to make them as intersubjective as possible, and conservation laws and their associated symmetries seem to have near-universal applicability to our observations, and relate in a deep way to the usefulness of maths in physics. Discussed here: The Unreasonable Ineffectiveness of Mathematics in most sciences
A lot of conceptual 'clumping' does implicitly involve self-reference by humans, as discussed here in relation to 'opposites': Life and Death as one and the same?
A case of particular concern for the specific subjectivity of humans affecting our understanding of the cosmos is time, where what we know about relativistic quantum mechanics, the Wheeler-DeWitt equation, is time independent. Quantum Field Theory seems to be almost entirely time-reversible, apart from CP Violation which is equivalent to violating T symmetry. Whereas, human experience has a strong time assymetry, linked to thermodynamics and the spreading out of information, which seems to imply that if time does move backwards, we would be unable to remember it. Discussed here: How does entropy explain consciousness and the forward direction of time?
"What is it that breathes fire into the equations and makes a universe
for them to describe?"
I take this as deeply wrong-headed, because I would follow a picture of physics as the emergent patterns of phenomena, that allow us to simplify our models by abstracting out what different situations share, and so make our models tractable. That is, existence before essence. There are lots of examples of complex behaviour being possible from simple systems, like say the Turing-Completeness of Conway's Game of Life.
All you need is the Uncertainty Principle, to begin: The universe was completely unable to tell if it was there or not, so it exploded. In the space of all probabilities, say something like E8 as options for deviations from a fundamental unity or simplicity, the Weak Anthropic Principle can account for the Fine Tuning Problem, of why the symmetries and constants are 'just so'. Like the presence of Many Worlds in Quantum Mechanics is considered by some ontologically 'wasteful', to violate Occam's Razor by multiplying conceptual entities beyond what observations require, so can be argued is the case for the realness of the 'super cosmos' of all possible physical laws. In both cases though, it may be about logical necessity, not meaningful realness, as say with mathematical notions of infinity, or the Tachyonic Field as indicating breakdown of a model. But alternately, there may be real existent places we can't visit because of the demands of the physics that sustains our biologies.
