Windows 10 version 21H1, will officially be build 19043. Microsoft is preparing its release with KB4598291, the patch that was issued to the Release Preview channel yesterday.
Windows 10 version 21H1 will be distributed to consumers in March or April 2021. KB4598291 will eventually appear as an optional update for Windows 10 2004 and Windows 10 20H2. As you may have already guessed, it is an enablement package that turns the mentioned versions into Build 19043.
The update contains the packages for switching to the 21H1 branch. Its registry entries also allow to identify it as build 19041.782, the first update for 21H1.
Microsoft-Windows-21H1Enablement -> Microsoft-UpdateTargeting-ClientOS vb_release_svc_prod3
The Spring update, version 21H1, will be a smaller one, like Windows 10 20H2 and 1909, which remind of what Microsoft used to call "Service Pack". Significance of the feature updates will be reversed. The bigger one with more notable changes, 21H2, will come in the second half of 2021. Check out the following post: