It moved over... DNS re-naming doesn't change the backend servers or databases, it only changes the name you address the site by.
Silly-but-accurate analogy: Think of it as the post office giving your house a new address... it's still your house, all your stuff is in it, just that after awhile, all of your mail has to have the new addy on it in order to show up in your mailbox.
What happens to user data on the old domain?
It moved over... DNS re-naming doesn't change the backend servers or databases, it only changes the name you address the site by.
Silly-but-accurate analogy: Think of it as the post office giving your house a new address... it's still your house, all your stuff is in it, just that after awhile, all of your mail has to have the new addy on it in order to show up in your mailbox.
Additionally, they moved authentication to a new domain for the benefit of all the communities recently (authenticat.win?)