Is "QGroundControl.Controls" a C++ class? And Where is it registed?

In MainWindowHybird.qml file, there is an ‘import QGroundControl.Controls 1.0’ line. Howere I cannot find where it is registed in cc/cpp files.
What does ‘QGroundControl.Controls’ mean?
Thanks for viewing this problem and waiting for your ansers and discussions.

That is a Qml Import for a set of controls defined by

Thanks for your reply, I have just found “QGroundControl.Controls” module definition in file “E:\Github\qgroundcontrol\src\QmlControls\QGroundControl.Controls.qmldir”
I wish Icould undstand qgc source code early.
Best regards!