To understand the internals of NS2, Pls read below book

1. http://www.mathcs.emory.edu/~cheung/Courses/455/Syllabus/A3-NS/Book/Introduction-to-Network-Simulator-NS2-2012.pdf

2. You can join Network Simulator 2 (NS2) google group wherein many questions and relevant answers are posted by researchers

Also take a look at this URLs

1. https://www.geeksforgeeks.org/basics-of-ns2-and-otcltcl-script/

2. http://mohittahiliani.blogspot.com/

3. https://www.codeforge.com/codes/search?search=ns2

First, under the basics of ns2. such as node creation and connectivity. Then work on protocols.

Use this cite .. https://ns2projects.org/ns2-simulation-code-for-aodv/

https://ieeexplore.ieee.org/abstract/document/9262397

Good question