There are 2 repositories under design-ui topic.
ReaLTaiizor is a .NET WinForms control library that offers a wide range of components and is user-friendly and design-focused.
A Flutter Design System 🎨 Example.
An application in Android Studio, which consists of the Graphical Interface of the Facebook application, created and developed in the Java language and its respective activities in xml.
Practice UI skills with UI Clones
This UI template ready to use and support your projects, you can pick some part you like and implement it into your code. All folder, file name, class name variable and function method is well organized and well named make this template easy to reuse and customize.