The whole history is pretty convoluted, since I'm fairly certain "OpenZFS" has been used to refer to several completely different source trees, with the Linux, FreeBSD, OS X, NetBSD, and Windows ports of ZFS all being separate forks off of the ZFS code from early OpenSolaris releases.
Despite the licensing drama and general distaste most distros and Linus have/had for ZoL, the Linux port had sort of become the de facto "standard" for ZFS development, with more work being done on that version of the source tree.
I only followed the "drama" from a distance, don't use ZFS or FreeBSD very frequently, but I know there was an internal discussion a while back within FreeBSD where they decided to pull commits directly from the ZoL tree instead of Illumos, because that tree was more active, and otherwise they had to wait on any improvements from Linux to be merged to the upstream Illumos tree.
The elephant in the room was the lack of activity in Illumos and the gradual decline of industry sponsors and development over the years. I don't know what the current state is, and I'm not saying Illumos is "dead", but it's definitely a shadow of its prime back when Joyent and a lot of companies were really pushing and developing on it. Several big names dropped out of Illumos development, including some NAS companies IIRC.
Anyway, OpenZFS 2, AFAIK, is essentially a rebranding of ZFS on Linux, but with CI/CD pipelines testing every commit against FreeBSD as well, basically unifying the work for Linux + FreeBSD.
The other versions (Mac, NetBSD, Windows, others?) still use separate forks AFAIK.