DeX is Samsung's proprietary facility that converts one of its Galaxy flagships into a desktop PC on connection to a monitor. One environment supported by the mode in Android 9.0 (Pie) is GNU/Linux. Unfortunately, its beta program will come to an end with the roll-out of Android 10.
https://www.notebookcheck.net/Samsung-discontinues-its-Linux-on-DeX-beta.439914.0.html