An example app that shows how to make a rounded button using Qt Widgets. This is trivially easy to do with QML, but non-trivial if you're using Qt Widgets.
It styles a QPushButton using Qt Widgets stylesheets to make the border and background rounded, then re-creates that same geometry in a QBitmap that is then set as a mask on the button widget. What the mask does is to both prevent painting in the areas of the mask that are Qt::color0 (0), as well as allow events to propagate to the widgets underneath it.
To illustrate that clicks will be delivered to widgets under the bounding box of the round button, but outside (inside?) the masked area, a standard QPushButton scaled to 1/4 of the size of the round button is placed underneath, to the bottom right on the window.
Install Qt from online installer or elsewhere. Open in Qt Creator, select Qt kit to use, build, and run. Should run on both Qt 5.x and 6.x.