I faced a similar challenge a few years ago; from my own projects to a large policy management system at an insurance company with literally 100s of thousands of source code files.

Of course it was extremely daunting at first, but what definitely helped me most was coming up with a strategy to find code where similar things had already been done.

This is easiest with a good IDE (search for aptly named classes, functions, etc.), and of course then asking somebody more experienced if you found the right bit of code. The last point cannot be overstated; most people love to show off their knowledge. If you come to someone with a question about a specific piece of code, they are usually extremely helpful. And importantly: if you don’t know who to ask, simply ask someone whom to ask! Again, make people show off their knowledge.