The unlocked Galaxy S9 and Galaxy S9+ are now being updated to Android Pie, which includes Samsung's One UI. A big change with One UI is that interactive elements are moved to the bottom of the screen, making it easier to use phones with big displays. Samsung has also added a night mode to One UI.
The Galaxy S9 and S9+ are getting the January 1, 2019 Android security patches with this update.
It's kind of strange to see the unlocked Galaxy S9 and S9+ getting Android Pie after the carrier versions because you typically think of updates for carrier-branded phones taking longer due to those carriers' testing processes. That's not the case with the unlocked S9 and S9+, though. The good news is that the updates have finally begun rolling out, though, so if you've got an unlocked S9 or S9+, be on the lookout for your slice of Pie.