Good morning Martin,
Thanks for this info as I learned a lot. How could I add my own widgets in a kind of library show in the QT Designer? Is it possible?
Yep, you can use your own widgets to Qt Designer by “promoting” a base widget type. You won’t be able to see them in the editor though, just once the application is running (Qt Designer doesn’t know what you’re drawing with Python).
There is a tutorial about using custom widgets in Qt here Embedding PyQtGraph (or any other custom PyQt5 widgets) from Qt Designer
Is that what you’re looking for?