What is Newton's law of gravity made of? What are numbers made of? What do mathematicians study if it's not matter?
Why must they be "made" of anything? I think you're making several errors here, starting with responding to claims I've neither made nor implied. I said, for instance, that matter and its interactions are the fundamental reality, not that everything is made of matter. You also appear to be confounding our descriptions of things with the things themselves, and failing to distinguish between the perceptual and conceptual, which leads you to ask questions that aren't meaningful.
Gravity, for instance, is a real thing generated by the presence of matter, we can feel its effects on us because we're also made of matter, and observe its effects on other bodies made of matter. That's perceptual. Newton's law is a conceptual description of it, which works pretty well most of the time, but it's turned out to be incomplete. Einstein's conceptual description of it is more complete, in that it correctly describes a wider range of interactions than Newton's version and correctly predicts things Newton's version doesn't, but we know it too is incomplete because it's not consistent with the other major theory of physics, quantum mechanics, which we also know is incomplete. But both general relativity and quantum mechanics are fully consistent with the claim that there is an objective reality that exists regardless of our perceptions of it or any conceptual models we make of it, and that reality consists fundamentally of matter and its interactions.
Mathematicians study ideas. Numbers aren't made of anything, they're concepts which, like many other mathematical concepts, have proven extremely useful in our attempts to create our conceptual models of reality, but they need not necessarily have anything to do with objective reality, any more than unicorns do. You might as well ask me what a unicorn's made of, it isn't a meaningful question to me.
You seem--correct me if I'm wrong--to lean toward the view that there is a realm of Platonic forms out there somewhere with an independent reality, where things like numbers (and possibly unicorns) exist. The main burden of your multiple questions amounts to "is that true?" I don't know whether it is or not, or what evidence could possible prove or disprove it, though it's often useful to think about things as if it's true. It's helpful, for instance, to be able to idealize a perfect circle whose circumference is a line of zero thickness exactly pi times its diameter, though we can't make one. This is a deep philosophical issue that much greater minds than ours have wrestled with at least since Plato's time, without a definitive result, and it's in mathematics I think that the really thorny questions appear.
Is mathematics implicit in the nature of the cosmos, or is it something we've invented to help us think about things? One of the most stunning results, at least to me, is what's called Euler's equation, a very short, simple, mathematical statement that connects five numbers,
e, the base of natural logarithms, pi, zero, one, and the unfortunately named imaginary number
i, the square root of -1, in a single elegant little equation:
e^
iπ +1 = 0. It's deeply connected to trigonometry, and thus geometry, and the same numbers show up repeatedly all over mathematical physics in contexts where there seems no obvious reason to expect them. To anyone with any sensitivity studying physics and mathematics, the immediate reaction is likely to be, "Why should this be so? Why are these numbers so intimately connected to all our descriptions of physical reality?" I don't know. I wish I did.