Hi Samia,
You should go through the documentation in Oracle EBS 11i documentation library.
https://docs.oracle.com/cd/B25284_02/current/html/docset.htmlFor the customizations, you should study forms development, custom.pll stuff, UI personalizations, Database objects & PLSQL, rdf report development and so on..
Normally, you should review the documentation of your own system. Those customizations were developed in your environment, so related documentation should be already available in your knowledge base. If not, then you should reverse engineer your system, but in order to be able to do that you should first study the things that I mentioned above.