I didn't even know they sold Google Domains. How does this kind of transfer work if I'm not a Square Space customer? Do I get a free transfer out of I don't want to use them?
I imagine it will initially be migrated to SquareSpace hosted services for your interaction that you then use your Google account to oauth into, so a pretty seamless transfer from the end-user perspective.
Likely the total transition will take over a year to complete. You will probably get a notification a few months after the transfer of control that the registrar name is changing and you dont need to do anything but make sure your contact info is up to date. At some point, probably 2-3 years down the line, domains.google will cease redirecting to SquareSpace domains.