In QtCreator click on Projects on the right side, then "Build & Run/Run".
"and in the system variable do we literally type "QT_OPENGL=desktop" in the field for the path" - there is no path. You simply add a new variable there (by clicking on Add button), change the Variable name to QT_OPENGL and Value to desktop, that's all.
Z-fighting is a completely different issue from aliasing. Maybe you need to set/check/increase the QSurfaceFormat::depthSize property (I've no idea what the default is).
If your z-buffer precision is as good as it can be, glPolygonOffset and/or controlling draw-order are next lines of defence against z-fighting.
I seem to remember the old forum allowed image upload; but maybe I'm thinking of the Jira (bugtracker). The markdown formatting here would let you link to a URL hosted elsewhere though (and maybe include it inline).