HTC released a nice infographic to depict the process of an Android OS update.
The Taiwanese company suggests that everything starts with Google releasing PDK (Platform Development Kit) to handset makers in order to evaluate the new framework. Soon afterwards, the search giant announces new version of Android and starts providing source code to the members of its eco-system, including chip makers and handset manufacturers.
If a chipset maker decides to support the new platform version on a particular chipset, they create the necessary drivers and make optimizations per that chipset.
From there on, HTC (and other vendors) assign resources to support development of the new software version via a maintenance release (MR). In the case of HTC, this is the point when it decides whether it will integrate the new platform with its Sense UI.
Then the work with carrier partners begins with the idea to add their customizations to the system.
Testing begins, involving a number of parties, including vendor itself, Google and mobile operator in case we’re talking about carrier-specific model. There are also unlocked / developer edition, and Google Play Edition devices, which get a slightly different treatment… Check how it all fits together from the nice infographic below…