The Win32 API is the name given to the original platform for native C/C++ Windows applications that require direct access to Windows and hardware. It provides a first-class development experience without depending on a managed runtime environment such as .NET.
This project consist in a portable image of Ubuntu that could run in Windows as a native win32 application. You could execute Linux applications and integrate it in your Windows desktop.
Alternatively called the Windows API and WinAPI, Win32 is the main set of Microsoft Windows APIs (Application Programming Interface) used for developing 32-bit applications.
Each major version of the Windows API has a distinct name that identifies a compatibility aspect of that version. For example, Win32 is the major version of Windows API that runs on 32-bit systems. The name, Windows API, collectively refers to all versions of this capability of Windows.
WinAPI (also known as Win32; officially called the Microsoft Windows API) is an application programming interface written in C by Microsoft to allow access to Windows features.
TL;DR: Use _WIN32, never WIN32. _WIN32 is automatically defined by the compiler, when you're compiling for Windows (even for x64; regardless of the subsystem choice).
Win32++ is an open-source C++ library designed to simplify the development of Windows API-based applications. It provides a modern C++ interface while staying lightweight and easy to use.
Through writing an application using straight Win32 you will gain a better understanding of how the API works, reduce bloat and have more control over the appearance and behavior of your applications.