Linux OS's have the Desktop, usually X11 based separate from the OS.
Some OS's have the GUI as part of the OS, ie Oberon.
Android uses Linux OS but the GUI is not X11 based.
You can make OS's without a GUI, FreeRTOS etc.
Don't get the look confused with the OS
We are just starting to see Linux OS's use Wayland/Weston instead of X11.
I have hopes the Pi4 will be powerful enough for Vulkan based GUI's
In the meantime I play with OpenVG/OS making, that's fun and possible now.
Lots for you to learn.
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges