As in, what are the concepts, constants, and interactions that have no known internal mechanisms that cause it?

Is the ratio of a circle to its radius pi for a reason? Would it be reasonable to expect the ratio be the same for all of existence?

Is there a way to predict the properties of hypothetical elements? As in, if we didn’t discover Uranium, would it be possible to predict that an element with 92 protons would be radioactive, can react in all the ways uranium is known to act, and has the spectral lines that uranium is known to have? Or is this unknown and
we, at the moment, have to treat it as a “just is” emergent property?