Darwin
Darwin is an open-source Unix-like operating system developed by Apple Inc., serving as the core foundation for macOS, iOS, iPadOS, watchOS, and tvOS. It combines components from BSD, Mach, and other open-source projects to provide a stable, secure, and scalable kernel and system services. As a POSIX-compliant OS, it supports multi-threading, symmetric multiprocessing, and advanced networking features.
Developers should learn Darwin to deeply understand and customize Apple's ecosystem, as it underpins all major Apple platforms, enabling low-level system programming, kernel extensions, and cross-platform development. It is essential for building system-level tools, security research, and porting Unix/Linux software to Apple devices, particularly in environments requiring tight integration with hardware or advanced performance tuning.