As I see it
OpenZFS (openzfs.org) is the name of a common roof to keep development on different ZFS platforms in sync. OpenZFS 2.0 is now also the name of a common codebase for ZFS developments.
OpenZFS repo was originally more or less a fork of Illumos minus the parts that are not related to ZFS as a filesystem (ex Solaris SMB server, Comstar etc). Some times ago OpenZFS (ZoL then) took over new features from Illumos.
Due the huge amount of developers on Linux, more and more newer ZFS features are now developped on OpenZFS on Linux first and no longer on Illumos what means that Illumos must take them over (ex encryption last year). The current relation of Open-ZFS 2.0 and Illumos is similar to Illumos vs OmniOS where OmniOS is a fork of Illumos that takes over newer Illumos developments from time to time (bloody) or from stable to stable.
As Illumos is not only ZFS but a whole integrated full featured OS with integrated storage services like SMB around ZFS, it may not as easy to use a common codebase directly like Free-BSD that requires no more features than Linux. But I expect Illumos to be as near to Open-ZFS as possible in future to make integration of features easy. Up to now, new wanted features from OpenZFS 2.0 are quite fast in Illumos (a few months with encryption, not the delay we have seen in Free-BSD of more than 1.5 years)