Here is actual code

DeviceDiscoveryDialog *DDD = new DeviceDiscoveryDialog(this);

where "this" is a parent of MainWindow

and the resulting WRONG test dialog is placed as top dialog

However coded this way
DeviceDiscoveryDialog *DDD = new DeviceDiscoveryDialog();

I still get the wrong dialog placed in center of main (parent) window.
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I am trying to find out from WHERE it comes from - irregardless who is the parent. .

In other words - what code goes into as DEFAULT parent , from where , and how can I add it as my own parent .

There MUST be a real code somewhere - similar to "this" pointer.
How do I code this "default" ?

In different words - I need SAME object with DIFFERENT parent and NOT same "default". AND I CANNOT FIGURE OUT THE syntax passing "widget pointer " if all I have is "form" pointer. .

DeviceDiscoveryDialog *DDD = new DeviceDiscoveryDialog(this);

Here this is your MainWindow, so it will be a dialog "owned" by your main window.

and "shows" default widget.

Not sure what you mean by this. DeviceDiscoveryDialog shows whatever widget(s) you have placed on it. That might be whatever you put on it in Designer if you did it that way. Or, if you did not use Designer for it and just created it in code it will be whatever widget(s) you added to it there. If you did not put any widgets on it it will just be blank. If you are asking how to add widgets now at runtime, for whatever reason, you would want something like

QVBoxLayout *layout = new QVBoxLayout;
DDD->setLayout(layout);
layout->addWidget(new QLabel);
layout->addWidget(new QTextEdit);
...

If you actually intend the DeviceDiscoveryDialog to be added as an MDI subwindow on your main window (I'm not sure whether this is the case) you would want something like

QMdiSubWindow *mdiSubWindow = mdiArea->addSubWindow(DDD);
mdiSubWindow->show();