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.
There are many articles, tutorials, references and FAQs which focus on component development. If this is your first time developing a component for Joomla, you should start with the Absolute Basics of How a Component Functions. If needed, you can visualise the control flow of a component with these diagrams.
Once you have read the tutorial and/or tried the example component, you can focus more on the specifics of your component with additional articles. These are listed on the Component Development Portal or any of the supporting Portals for Development(Plugins, Modules or Templates).