This article discusses software internally used by Apple.
Acquiring a copy without Apple's consent is illegal and may result in being scammed.
All tests are performed via a usbmux interface. During that time the iPhone displays a picture of a fireball. Partial output is displayed at the bottom.
However when display tests are run, different output is produced.
After booting up, SwitchBoard.app is shown, allowing you to choose multiple different applications.
They consist of:
The firmware includes standard set of UNIX binaries. apt-get is present suggesting that there is an internal APT repository.