Qt uses theme engines (written in C++) to change the appearance of widgets. It also has platform plugins which are used to set the theme to match the current desktop (Gnome, KDE, windows, mac, etc), and also to provide access to the platform’s file dialogs, colour picker, etc.
Previous Ubuntu releases used QGtkStyle for the theme. This used Gtk2 to paint into pixmaps, which were then drawn where the Qt widgets would be. This was never that good - arrows in scrollbars were sometimes missing, or not drawn correctly. For the platform plugin, there was a dbus menu plugin to export a Qt app’s menubar to the global menu bar, etc. Again, this had issues where it always drew menu icons for Qt apps.
Fedora currently use the Adwaita-Qt theme engine - which is a C++ engine specifically mean to mimic Adwaita. They also use the QGnomePlatform platform plugin - which tries to locate a Qt theme that matches the current Gtk theme (it can use Adwaita-Qt for Adwaita, and Kvantum for others). This platform plugin also provides access to the Gtk3 file dialogs.
As I’ve stated before, the Kvantum theme engine can be used to provide a much better Gtk-like theme for Qt apps. This is a C++ engine, which loads SVG files to draw the widgets. It also provides Qt apps with real overlay scrollbars (the only Qt engine that currently does this). For an example of a Qt app using Kvantum’s Ambiance-like theme, see Cantata This is not 100% perfect, but is much better than the experience provided by QtkStyle
To provide a Qt equivalent for whatever Gtk3 theme is chosen, should be a matter of providing a Kantum config and SVG file for this theme. (I’m 99.99% sure Kvantum’s author would be more than willing to help). The QGnomePlatform should also be used so that Qt apps automatically pick up a matching theme for the user’s Gtk theme.