unix:!macx: LIBS += -L$$OUT_PWD/../../../../../../../../../../home/qe/Qt/Examples/Qt-5.15.2/bluetooth/btscanner/ -lbtscanner

INCLUDEPATH += $$PWD/../../../../../../../../../../home/qe/Qt/Examples/Qt-5.15.2/bluetooth/btscanner
DEPENDPATH += $$PWD/../../../../../../../../../../home/qe/Qt/Examples/Qt-5.15.2/bluetooth/btscanner

Then I add #include "device.h" to my MAIN subdir main.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

**#include "device.h"**

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    DeviceDiscoveryDialog *DDD = new  DeviceDiscoveryDialog();
    DDD->show();
    // test sub dialog
//    TEST_SUB_Dialog  *TSD = new TEST_SUB_Dialog();


}

Checking the added #include "device.h"

#ifndef DEVICE_H
#define DEVICE_H

#include "ui_device.h"

#include <qbluetoothaddress.h>
#include <qbluetoothdevicediscoveryagent.h>
#include <qbluetoothlocaldevice.h>
//.include <qbluetoothlocalde#vice.h>
//#include <QBluetoothLocalDevice/qbluetoothlocalde#vice.h >
#include <QDialog>

I find another missing #inlcude(s) - however using "follow symbol under cursor" I can open the "missing #include <qbluetoothlocaldevice.h".

Obviously suddirs failed to add dependencies for "device"

1. what is the proper - preferably using wizards - way to add such dependencies to "device" ?
2. Why can I open the missing depended (downstream ) include but compiler cannot find it ?
3. This is what "device .h include hierarchy " tells me