Comment by DrinkyBird

KDE has the newer QML-based Kirigami and older Qt Widgets frameworks that are not consistent. Widgets apps absolutely look like actual desktop apps (menus, toolbars, dialogs), Kirigami apps look like mobile apps (nav bars, hamburger menus, page-based navigation). There is definitely a visual and functional inconsistency between the two, even if they use the same theme pretty well.