What do you mean by "single OS" and what is the basis for your belief?
All Apple operating systems (macOS, iOS, iPadOS, watchOS, tvOS) — are all forks of OS X and share most of their codebase. The main differences are the user-facing framework (which is optimized for the type of the device) + some tweaks to make sure that the OS runs well on the device. If you want, all these OS variants are like specialized distributions of the same OS that ship with certain libraries and configs depending on your deployment target. You can compare them to Linux distributions (although differences between different Linuxes can be much more dramatic than between macOS and watchOS for example).
If you say "single OS", you are implying that Apple wants to make one OS that will be bootable on every device. This will not happen, simply because of different device capabilities. The iOS devices will always have less memory than their Mac counterparts — it would be pointless to install stuff on them that is specific to the Mac for example. If instead by "single OS" you mean that the Mac and iPhone system software is identical — well, this has been the case for the last decade.