From Joomla! Documentation
The development of Joomla! itself is carried out by the Production Working Groups and third party developers. This page is a starting point for developers interested in the development of components, modules, plugins and templates.
The development of Joomla itself is carried out by Production Working Groups and third party developers. For more information about Joomla! core development, how to contribute to code to core, or becoming more involved with the core code, you should go to Joomla! Code Contributors.