This is an older video is a brief introduction about Linux development. Some of the statistic are out of date, notice it is from 2012.
How Linux is Built
"While Linux is running our phones, friend requests, tweets, financial trades, ATMs and more, most of us don't know how it's actually built. This short video takes you inside the process by which the largest collaborative development project in the history of computing is organized. Based on the annual report "Who Writes Linux," this is a powerful and inspiring story of how Linux has become a community-driven phenomenon. More information about Linux and The Linux Foundation can be found at http://www.linuxfoundation.org and http://www.linux.com"