Ideally, execute_evcxr should cease to exist. It's purpose is to compensate for shortcomings of evcxr kernel. Consequently, spending too much time on the project is not recommended.
There is, however, a relatively non-dev-friendly STRUCTURE.md that exposes such details as what is the responsibility of each file in the source directory. In the ideal world, STRUCTURE.md should be machine-generated from some src_description.json. However, it's not and it can get outdated really fast. At the moment of writing, it is.
Note: STRUCTURE.md was born because the girlfriend of the author was curious what it's like to code and the author found no better way to explain how he works on execute_evcxr, as well as how execute_evcxr does its job.