Kirigami
Kirigami is a set of QtQuick components for building adaptable UIs based on QtQuick Controls 2.
Its goal is to enable creation of convergent applications that look and feel great on mobile as well as desktop devices and follow the KDE Human Interface Guidelines while being easy to use and not adding many dependencies.
Kirigami works on a variety of platforms, such as Plasma Mobile, Desktop Linux, Android, MacOS, and Windows.
A tutorial for Kirigami is available on the KDE Developer platform.
A list of additional QML learning resources is available in the Community Wiki. If you are not familiar with QML at all, the QML book should be a good head start.
If you have any questions about Kirigami, feel free to drop by the Kirigami group on Matrix.