On March 6, Microsoft announced that they are open sourcing the Windows Calculator application on GitHub, which will be available under the MIT License. That means, you can now contribute to the development and reuse the already available code.
As part of the MIT License, Microsoft is open sourcing the code, build system, unit tests and the product roadmap for the Windows Calculator. The project is available as "clone-and-go" and the development will follow the standard GitHub flow.
That means, you can now directly contribute to the development of the Windows Calculator in GitHub, participate in discussions, report issues, suggest new features, prototype new features and fix bugs.
Apart from contributions, those who are interested, can use the Windows Calculator source to learn about the Universal Windows Platform (UWP), XAML, Azure Pipelines and Fluent Design.
The source code of the Windows Calculator is available here on GitHub.