Explain the order and the logic of using various SSL backends

Many users reported and keep reporting to Qt support that it is no clear how Qt decides which SSL backend to use at runtime. It is also not clear when using pre-built Qt from Qt Installer. Apparently, some fallback logic allows using an alternative backend if the default one is not available at runtime. Please provide some documentation how all this works so that users can better understand what is going on and apply some adjustments