Microsoft has been testing Xbox One 20H1 since September of last year. It first started out in the Preview Alpha - Skip Ahead ring, but it didn't get any new features for a while because Xbox One consoles get monthly feature updates. Anything that showed up in 20H1, which is version 2004 (Microsoft is much more upfront about Xbox version numbers than with Windows), also showed up in the minor monthly update.
But now, the next update is actually version 2004, and it's been promoted to the Preview Omega ring. To draw a parallel between the Xbox Insider Program and the Windows Insider Program, this is like the Release Preview ring. It means that Microsoft is prepping for release.
Ever since the release of Windows 10, PC and Xbox OS development have been done side-by-side. Xbox One consoles run a flavor of Windows 10, which is why the build numbers are always the same. Xbox One version 2004 will be build 19041, just like Windows 10.
But while this is a major update in terms of how it's shipped, it's not so much in terms of features. There was a time when the Xbox updates that shipped alongside Windows updates were big bang feature releases, with minor updates in-between. This has evolved though, with every update adding a few features here and there.
Microsoft hasn't published a full list of features that will be included in Xbox One version 2004, but it will likely do so soon.