How do GTK and Qt integrate with Linux in comparison to how they integrate with Windows and OS X?

Question

From my understanding, Qt and GTK on the Windows and OS X side are just wrappers around the native GUI libraries, like for OS X it wraps around Cocoa, and for Windows around

Answer 1

A few further notes.

Neither GTK+ nor Qt use the native widgets of Windows and OS X. They approximate the look and feel using native APIs, but internally everything is all done custom.

GTK+ and Qt are responsible for, and define, the themes available to programs on Linux. Desktop environments typically provide a way to change the theme globally for all applications, but how this is done is defined by GTK+ and Qt. For example, GTK+ 3 typically uses ~/.config/gtk-3.0/settings.ini to store this information (and there is a programmatic API to this file).

Qt has a bridge for GTK+ 2 themes via QGtkStyle, and the KDE developers maintain versions of their Oxygen theme for GTK+ 2 and GTK+ 3. (The previous sentence may change in the future, especially now that GTK+ 2 is long dead.)

Update 1: Unix systems only provide a way to reserve a rectangular region of the screen to do what you want with it, including drawing (as in plotting a bitmap image) to it. Drawing (as in drawing shapes) is done by hand. GTK+ uses a library called cairo to do its drawing; I believe Qt wrote their own (QPainter?). Both Windows and OS X provide drawing APIs (Windows has several; OS X has Core Graphics). (X11 does have drawing primitives, but I assume they are not expressive enough to be used for modern 2D graphics; I wouldn't know...)

The same applies to font rendering, though modern Unix systems tend to base their font rendering on some generally accepted base libraries (freetype, fontconfig, fribidi, harfbuzz). GTK+ uses Pango to do text layout (actually arranging blocks of text into lines and paragraphs) and drawing (Pango integrates with cairo); I believe Qt also uses its own (this time I'm not sure).

I wrote about what X11 does do some time ago.