@OldRoly
Qt Creator is just a standalone IDE, it does not bring any Qt packages with it for you to use to build your own programs, technically it could be used as e.g. a text editor without anything to do with Qt development (though you would not want to).
I don't know why you ask for the "complete package" and then mark this topic as Solved with no further comment. To build your own code you need to fetch some of the distro packages. I can't recall just what they are called, and it can vary from distro to distro or release to release, but you need to type something like sudo apt install qt and then press the Tab key once or twice to see the offered completions. You will probably want things starting from qt6 and then Tab, there something like qt6-dev with most of what you will want? You can Google for what exactly the names are and what might be wanted, but as I say the names change so it may not be accurate for current your distro.