Ask HN: How to dive into large codebase?

I have seen lots of tutorials and posts teaching about different programming languages and frameworks and stuff but I have never seen a tutorial teaching about how to dive into large codebase. I am somewhere in between beginner-intermediate level programmer and I have always wondered how programmers familiarize themselves with large codebases. There are tons and tons of open-source projects on GitHub, SourceForge, Bitbucket and ... you name it. Most of them have contributers who starts contributing after project matured and it doesn't seem they would have red and understood whole source code of project before contributing or have they? Now my question is:

Are there any tutorials,books or other materials teaching about how to dive,familiarize,read,understand and contribute to large codebases efficiently?