qt5_add_resources(RESOURCES modules/ProjectName/resources.qrc)
That thing converts qrc files into C++ files. Names of C++ files are stored in the RESOURCES variable.
Executables and libraries are compiled from C++ files by using add_executable and add_library in CMakeFiles.txt. In the "QML app with C++ plugin (cmake)" project template the "C++ plugin" part means library. So somewhere in CMakeFiles.txt there is an add_library. Resource files should be added there:
add_library(Myappbackend MODULE
${Myappbackend_SRCS}
${RESOURCES}
)
Alternatively, the qrc can be compiled directly into separate library by the qt5_add_resources macro or not compiled at all (so no qt5_add_resources at all) and be load by the Qt application in runtime (see source of the core ubuntu-terminal-app for example).