how to understand big new project code quickly and effectively

how to understand big new project code quickly and effectively to write new enhancement to existing code or to fix some defects or to debug to improve some areas of poor performance.

what are the tips and recommendations , best practices around this
Please advise
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I used Understand by SCI Tools on large C and C++ projects. As co-workers saw what it could do, they started using it, and we kept buying additional seats. It's great!

They support Java and a number of other languages.

The license is relatively inexpensive (at least 10 years ago it was).

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System Programming

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