PySide6 is a Python binding of the cross-platform GUI toolkit Qt. Applications built with PySide6 will run on any platform supported by Qt & Python including Windows, OS X, Linux, iOS and Android.
PySide is the official binding for Qt on Python and is now developed by The Qt Company itself.
For a complete guide to desktop application development with Python & PySide6, see the PySIde6 tutorial and PySide6 book Create Simple GUI Applications with Python & Qt.