Mostly based on Qt responsive Framework Kirigami https://api.kde.org/frameworks/kirigami/html/index.html .
The GTK counterpart on Gnome Phosh (Phone Shell) was libhandy (https://gnome.pages.gitlab.gnome.org/libhandy/), now for GTK4 merged to libadwaita (https://gitlab.gnome.org/GNOME/libadwaita).