New subscriber - looking for an overview of the code

Hello devs,I am a coding enthusiast. I have downloaded code in a Ubuntu VMWare machine and I am able to compile the code.
I am looking for a course / site that can give me an overview of the code like:1. How is the code organized and which file/folder has these components?     * Core RDBMS engine components,      * additional DB components,      * DB Rule based security components       * DB Storage components    * Index storage
2. Is there a code sample that can demonstrate DB engine changes (like a sample to change SELECT to SEL or SLCT, or adding a new mathematical operator)
Thanks
Lateef