It looks like Microsoft many have accidently pushed out a build of Windows 10 20H1 to all Windows Insiders. The build is an internal test build and there reports of it going out to Release Preview, Slow Ring and Fast Ring. So far I have seen it show up on a 32bit Fast ring and it was offered to a device running 19H2 on the slow ring so pause you may want to pause your updates for now.
I will update the post when we have more details.
UPDATE 17:00 Microsoft have pulled the update. If installed the build you can roll back to your previous build. No word from Microsoft on what happened.