This is the Qt version for the qmake command found in your PATH environment variable. This is likely to be the system's Qt version.
The version of Qt configured in the Projects mode, Build Settings, Qt Version field. This is the Qt version that is actually used by a particular project.
Shadow building means building a project in a separate directory, the build directory. The build directory is different from the source directory. One of the benefits of shadow building is that it keeps your source directory clean. Shadow building is the best practice if you need many build configurations for a single set of source.