Just like when Windows XP was developed: they took the Windows NT 4.5 codebase, and started hacking on that until they'd changed enough to make it "Windows NT 5", but it was never released under that name. The underlying version number, however, was kept. So, it'll probably be Windows 11 (or rather, a Windows "something") with an underlying version number like 10.5.x or 11.x.x
They realized at some point that they had several concurrent versions of Windows out there that they had to support, and many customers had no apparent incentive to upgrade to new versions. So going forward they ostensibly have a single version that auto-updates, and they provide the OS "as a service".
Right now there are a bunch of little forked differences between them all. Microsoft says they are working to merge all the shell differences into a single project (CShell; composable shell), and the point of getting to a clean core (the "Polaris" codename in this article) seems to be to make sure all the rest of Windows modularly slots in on top of that, and a benefit to that is that you could quite possibly more easily change between which modules are loaded/available.
From the Continuum project, we know that Microsoft finds it interesting to be able to switch between Mobile and Desktop editions as much "on the fly" as possible. Could mean they have ideas for switching between Desktop and Xbox modes, too.
The clean core is Windows Core OS; Polaris is the desktop-specific composer on top of CShell and Windows Core OS and is also used in this article as the name of a UWP-only Windows edition incorporating the Core OS/CShell/Polaris-composer stack.
It makes sense that there would be a codename for the desktop-composer on top of CShell, and that would be a far better explanation of things than the article here.
https://www.windowscentral.com/windows-core-polaris
This Windows Central article actually seems to be the source for most of the other recent coverage, so it may be the most complete piece out there.