When working on a large system, it can be difficult to find which portion of pre-written code does what you want. Keep digging as if you are searching for treasure, somewhere in the code there is a class, a method a function that does what you need. Don’t re-invent the wheel, chances are its already been done.
What if you have to write new code? Look for patterns (interface-implementation, class centric design, procedural/structural code in methods).