SDL is not a GUI library: it doesn't have any buttons, sliders or text controls. That's what this library provides.

To get on something on screen, it provides a number of backends, including SDL. However there is also one that talks to X11 directly, providing the minimum dependencies needed on Linux (as far as I'm aware).

Basically the library generates a list of drawing commands. It is up to the underlying backend (SDL, GLFW, X11, your own embedded one,....) to draw these on-screen.