Prior to the release of the Galaxy Note 9, you could only access Samsung's DeX desktop-style UI using a compatible dock.
Galaxy Note 9 users can get DeX's desktop experience with an HDMI to USB Type-C cable and a monitor. That allows them to control the action on screen with a bluetooth mouse and keyboard.
This feature is now available to Galaxy S9 users via Android Pie and you can take a look at the feature in action on the Galaxy S9 in the video below.