In short, Apple designs very few custom parts, designs their boxes, writes some software and outsources manufacturing.
Apple _makes_ no hardware, it is all outsourced to third party companies. That goes for the individual parts inside each of their computers too. They do design _some_ low level parts along with their Macs, iOS devices and other miscellaneous hardware, all of which get manufactured by other companies.
Apple also writes their own operating systems and applications they release. I'm not sure it is complete yet, but Apple is moving to an online distribution model. When they distribute software on disc, they have third parties produce that physical medium.
