The concept of "if you want to touch this code base you have to understand the entire thing" is pretty tough on basically everybody but the people who either wrote most of the code or who live in that code base all the time. It would seem like code-bases should (either because people architected it to be possible to work with without understanding the whole thing or the programming language helps facilitate it) be able to be worked with while only understanding a portion of the code base. In some cases classes and interfaces help with that, but in plenty of cases you end up in one of the sort of child classes trying to figure out how to get out of the hole you are in where you can't interact with anything anywhere in the code base and you have to basically plumb pipes through lots of places to allow moving data where you want.