Wozniak revealed that there were lots of trials involved in the  making of products, all of which took place in secrecy. “Steve Jobs kept  a lot of products secret when he returned. If everyone knows what  you’re doing it makes you scared.”
“Now the iPod wasn’t let out until it was so obvious that this was an  extreme step different to everyone else’s music player,” he said.
“People like things that are easy. With iPhone, the secrecy allowed  us a lot of ways to look at it. Apple developed other phones, for 6  years, but the problem is, they just didn’t have that special gleam,”  said Wozniak. “That’s what lead Apple to finally recognising that once  we’ve solved the problems with the iPhone, we had a product that the  world had never expected to see.”
Wozniak said that when they first started Apple, “Steve Jobs was  worried that the big companies will jump on us. They’ve got more  resources.”
http://www.macworld.co.uk/news/apple/wozniak-reveals-why-steve-jobs-loved-secrecy-3357801/ 
A former Apple intern identified by Business Insider only as “Brad” recently recounted his time with the company, and he  shared the story of one Apple employee who worked on the iPad’s display.  Though he was on the team responsible for this key iPad component, the  employee had absolutely no idea the iPad was being developed, nor did he  know why he was working on a 9.7-inch display.
“They didn’t know if it was a big phone or a small laptop,” said  Brad. “They had no idea. It wasn’t until the product release where Steve  Jobs went on stage and showed the iPad that they realized this is what  we worked on for the past two years.”
He continued, “It’s super crazy that they can make this work, because the different teams are siloed.”
http://bgr.com/2015/07/02/apple-secrecy-steve-jobs-ipad/
Apple accomplishes this by closely monitoring work spaces, making  developers chain products to desks, and even requiring employees to  cover up devices with black cloaks while working on them.
http://www.businessinsider.com/the-most-extreme-examples-of-secrecy-at-apple-2013-7