As you said, existence is constrained by laws of physics of this universe and by potentially different laws/constraints in other universes that we can’t possibly know. Because we lack the ability to observe the infinite expanse of universes as a whole, we can’t know what the constraints are (if any) so we can only understand them as infinite possibilities without constraints, but this doesn’t mean that there are not constraints that we’re just incapable of observing, and I feel like this was OPs point with this post. We don’t know what we don’t know.