The operating systems and their relationship.
WHAT THE OPERATING SYSTEMS ARE
macOS, for computers iOS, for phones iPadOS, for tablets watchOS, for watches tvOS, for televisions visionOS, for spatial devices
WHAT THEY SHARE
A common foundation, derived from a Unix base Many of the same frameworks One development environment One developer account
WHAT THAT MEANS PRACTICALLY
Skills transfer between them, and code frequently does.
WHAT DIFFERS
Interface conventions
What is permitted: macOS is far more open than iOS
Distribution rules Hardware capability
WHAT THE UNDERLYING SYSTEM IS
A Unix-certified system, with a command line, a standard file hierarchy and standard tooling.
WHY THAT MATTERS
Server, scripting and development knowledge from other Unix systems transfers directly.
WHAT SITS ABOVE IT
Apple's own frameworks, which is where the platform's character lies.
WHAT TO UNDERSTAND FIRST
The distinction between the Unix foundation and the Apple layer above it.
