If you want to draw in "map coordinate" space, then I'd suggest using our Geometry classes to create a Point, Polyline, or Polygon, and then display it on the map with a Graphic. That is showcased in this sample - arcgis-runtime-samples-qt/ArcGISRuntimeSDKQt_CppSamples/DisplayInformation/GOSymbols at master · Esr...
If you want to draw in screen coordinate space and it doesn't need to stay tied to a map location, you can use any of Qt's mechanisms to draw, like QPainter.
