@danct12 Qt and GTK place them in opposite directions by default, and of course some devs write their dialogboxes from scratch and don't care which goes where :) If you use qt5ct you can have it place Qt dialog buttons in either the Qt or GTK fashion for consistency across your system.
@nilix and @danct12, I'd like to add that in case three buttons (Apply, Cancel and OK), there are at least four conventions in total: Windows, macOS, KDE and GNOME.