Its starting Samsung is rolling out a beta version of the One UI update to Galaxy S9 and Galaxy S9+ phones in South Korea. This update brings with it Samsungs revamped interface built on top of Android 9.0 Pie, plus several other improvements.
Samsung DeX is free from the dock and can be used with any USB-C to HDMI adapter. Another neat change is that you can tune the cameras color rendering to your liking.
Change log for Galaxy S9+’s Android 9.0 Pie update (in Korean)
Note that some of the Samsung apps will need to be updated through the Galaxy App store. They have new functionality too. For example, Samsung Pay now has a new interface to manage debit cards. Also, Samsungs file browser can analyze files in the phones storage and help you clear out the cruft (similar to Googles Files app).
Its a pretty massive upgrade, about 1.6GB, so it will take a while to download and install. Again, this is happening in Korea right now, but should expand to other regions.