Thanks for your answer. So if I understand correctly:

In my case, I will use several already made Python classes/codes to control the devices, the GUI being mainly an interface between the user and those codes. If I want to create a proper application which runs without having to find pieces of Python code, I would have to use Qt Creator, make it such it can use Python code (as far as I understood, this is possible) and create a new project with all the Python code that I need to use in my GUI. Finally, I would have to compile it. On the other hand, if I use Qt Designer, it will make a .ui file that in have to transform into Python code with something like pyuic4. Then, it would still use the Python classes/codes.

Am I correct?