What happened to "Insiders would be the first to get the upgrade?"
I got my Plex Server to upgrade, but only by using the Media Creation Tool. Seems that may be what I have to do with the remaining 3 machines since Windows Updates doesn't seem like it's going to fly.
What I found says the WINDOWS.~BT folder is (as I think we all already knew) is where the Windows installation files get downloaded to. Anyone doing a Windows 10 Upgrade will get this folder (actually this isn't just specific to 10 upgrades). If you run the system files cleaner afterwards with all boxes checked this gets deleted along with windows.old
If you upgrade using the Media Creation Tool you will also get the Windows.~WS folder. I don't know if you get this when upgrading via other methods, but I've never seen it before. Anyways the article says this contains the necessary holes to roll back the upgrade should it fail or other need to be rolled back. This folder does not automatically get deleted during the system files cleanup because once you delete it a rollback is impossible. But you can manually delete it once you're certain you won't need to rollback.