How X11/Qt/Gtk/ implements scrolling

How X11/Qt/Gtk/ implements scrolling

I hope someone can help me with this: how X11/Qt/Gtk implements scrolling?
I do not want to know how to use their APIs to use scroll on a program,
but how the APIs implements scrolling.
For instance, the program receives a request to redraw itself because the
user had scrolled only a single line down/up. Do the program redraw only
this single line?
Does it uses blit operations? 3D and/or 2D acceleration? Most important:
where can I find references about this subject?
This question could also be: how to implement scrolling in your homemade
windowing system.
If someone needs to know: - custom operating system made from scratch with
assembly/C - video using VGA