iOS was forked from MacOS more than a decade ago. Of course some code is shared between them, but they do not have the same base in any sense of the word.
It’s really more than “some code”: the kernel, core frameworks, and many utilities are a single codebase with platform-specific #defines. I have no idea if there was an actual fork when iOS was being made, but if there was they merged the changes back in.
True, but there are a lot of elements of the system that are not shared but are pervasive, and so the assumption that the bugginess of ios13 somehow must also be in Catalina is not valid.
