That relates to another misunderstanding. These ideas like "everything is a function" or "everything is a category" are not there to help you understand complex system better.

They are supposed to help you build better foundations; the abstractions to better understand your complexity you need to build (or at least pick) yourself.

Good foundations then help you relate the abstractions. For example, the categoric dual of a product is a sum. We can apply it to relation (relational algebra), and we get that sum is data inheritance.

So that gives you understanding of how functions (foreign keys), product (relation) and sum (inheritance) fit together. This sometimes helps to build abstractions in a consistent way.