How to use qmake.conf?
qmake.conf, but how can I use it? What is the format? What would be one use case for it?
qmake.conf as the name suggests (and as the documentation you link says) is a file containing persistent build configuration information for your project. Each toolchain will contain a mkspecs directory with a series of subdirectories for each target that toolchain can build for.
You generally don't need to modify qmake.conf yourself. If you're using qmake for your project, you will have a .pro file in which you can put whatever you need. Generally, Qt Creator's project wizard will give you a good start.
Note that if you're using CMake, QMake is irrelevant.