Too Long; Didn't Read
A large code base can be overwhelming. Instead of trying to get to know the whole thing, I tend to focus on the problem at hand. This gives me a safety net. It does not guarantee that my changes are safe, but it helps a great deal. If that simple change causes something to break, I will note what broke, and move to the area that was broken by my change to see if I can get tests around it and prepare it to accept my change. Take a look at Mikado Method for more on this approach to changes to large systems.