How to start customization into code level

I have installed devstack and koa.master branch. Now i want to know the from where i have to update or customization. How to understand openedx core. etc .

Please suggest me any tutorials for beginner level code modification.

hey
have you seen these documents?
https://open.edx.org/community/develop/
https://edx.readthedocs.io/projects/edx-developer-guide/en/latest/