Is there any documentation for the code base?

I’m a security researcher on Gitlab’s bug bounty program, this may sound weird, i would like to know if there is any documentation on the actual codebase of Gitlab?
for example if i spot a code snippet while browsing the code, is there any documentation that can tell me how do i access this piece of code in the ui ? or the opposite, lets say i’m testing Gitlab through the UI (in the browser normally) and would like to know which piece of code is handling my input etc, is there such thing ? if not if there is any advice on this matter or anyone that i familiar with the codebase i could generally ask questions that would be great as well