Forstall was also responsible for creating a software development kit for programmers to build iPhone apps, as well as an App Store within iTunes. The decision enabled the success of the iPhone as a platform for third-party developers: using a well-known desktop operating system as its basis allowed the many third-party Mac developers to write software for the iPhone with minimal retraining. Jobs favored the former approach but pitted the Macintosh and iPod teams, led by Scott Forstall and Tony Fadell, respectively, against each other in an internal competition, with Forstall winning by creating iPhone OS. In 2005, when Steve Jobs began planning the iPhone, he had a choice to either 'shrink the Mac, which would be an epic feat of engineering, or enlarge the iPod'. The current stable version, iOS 17, was released to the public on September 18, 2023.

Major versions of iOS are released annually. These mobile apps have collectively been downloaded more than 130 billion times. As of March 2018, Apple's App Store contains more than 2.1 million iOS applications, 1 million of which are native for iPads. Unveiled in 2007 for the first-generation iPhone, iOS has since been extended to support other Apple devices such as the iPod Touch (September 2007) and the iPad (introduced: January 2010 availability: April 2010). It is proprietary software, although some parts of it are open source under the Apple Public Source License and other licenses. It is the basis for three other operating systems made by Apple: iPadOS, tvOS, and watchOS. It is the world's second-most widely installed mobile operating system, after Android. It is the operating system that powers many of the company's mobile devices, including the iPhone the term also includes the system software for iPads (predating iPadOS, which was introduced in 2019) as well as on the iPod Touch devices (which were discontinued in mid-2022). IOS (formerly iPhone OS) is a mobile operating system developed by Apple Inc. Proprietary software except for open-source components

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