This project has an example OpenGL/GLES based program that has been tested on Windows 10 Pro, Ubuntu 18.04.03, macOS 10.15.1, the iPhone simulator 11.3.1 and Android 10. It can serve as a starting point to build other cross-platform applications.
It includes constructing appropriate headers for vertex and shader programs to work with either OpenGL or GLES and integrating the OpenGL widget into a GUI that has other widgets to control it.
It is released under the BSD 3-clause license, but of course a Qt license is also required to build it.
The project can be found at https://github.com/ReliaSolve/qt-cross-platform