Would it be acceptable to re-post the "update" ?
Upon further research I have found than the failing project ( all in 5.15 ) is actually part of subdir project,
and my guess is that that is why it worlds by itself but fails when added as subdir.
The "bluetooth.pro" "main" suproject actually consist of two more sub protects.

All of them are still "only" in 5.15 - no Qt 6 at all

I've figured it out: the trick is to use the menu item "Build for run configuration..." and assign that the Command+B keystroke.

@Tee_
normally a pro file results in a separate target (executable or library). Even if you "combine" them with a subdirs project.
So your AppUi project must link against the AppLib

@rtavakko I'm so pleased to hear the working solution! Thanks for sharing the good news :)

Hi,

What are these "minor things" ?

After few days searching on Google.
I found the answer.

The problem is the order you Link to Static Library.

Dependency of my Projects is:
Presentation << Utilities << DynamicObjectCore

in Presentation.pro, I'm link to Static Link Library in this order

# Link to DynamicObjectCore ... # Link to Utilities ...

I thought that it will compile DynamicObjectCore first, then compile Utilities.
But it wan't, and cause the compile error. "undefined reference to..."

When i change the Link Order

# Link to Utilities ... # Link to DynamicObjectCore ...

And it work.