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How do you dive into large code bases?

I have quite a bit of experience with different programming languages and writing small and functional programs for a variety of purposes. My coding skills aren't what I have a problem with. In fact, I've written a decent web application from scratch for my startup.

However, I have trouble jumping into unfamiliar applications. What's the most effective way to approach learning a new program's structure and/or architecture so that I can start attacking the code effectively? Are there useful tools for their respective languages (Python and Java are my two primary languages)? Should I be starting with just looking at function names or documentation? How do you veterans approach this problem?

I find this has to be with minimal help from coworkers or contributors who are already familiar with the application and have better things to do than help me. I'd love to practice this skill in an open source project so any suggestions for starting points (maybe mildly complex) would be great too!