That again may be implementation detail, and may vary upon toolkit version and target platform.
In general, Qt encourages use of QBrush and QPainter to draw on QWidgets. A lot of shape-drawing operations is abstracted; application code supplies coordinates and drawing style and platform is supposed to do the actual operation. Another versatile widget is QGraphicsScene (along with QGraphicsView) which works in terms of sprites and probably maps pretty well onto 2D acceleration of graphic cards.
Under very heavy load you can sometimes see steps of Qt widgets drawing -- it's usually frame by frame (as in, rectangle by rectangle), rather than pixel by pixel.
In general, Qt encourages use of QBrush and QPainter to draw on QWidgets. A lot of shape-drawing operations is abstracted; application code supplies coordinates and drawing style and platform is supposed to do the actual operation. Another versatile widget is QGraphicsScene (along with QGraphicsView) which works in terms of sprites and probably maps pretty well onto 2D acceleration of graphic cards.
Under very heavy load you can sometimes see steps of Qt widgets drawing -- it's usually frame by frame (as in, rectangle by rectangle), rather than pixel by pixel.