How do you understand the structure of large open source projects

[mydarkpassenger]

I've downloaded some open source projects and frankly the amount of files in them is overwhelming. I've only ever worked on my own projects and I don't understand how you would know how to navigate large projects or do any modifications on them. Are there any tricks out there for navigating and understanding large projects?

[Tolls]

Run them.
Then follow a flow of code from (say) a button push and see if you can trace what it does. Debugging can be handy in this.
This assumes the documentation is rubbish of course, which is sadly often the case.

[mydarkpassenger]

Thank you, I wasn't sure if there was a way to understand a program structure that I wasn't seeing. Didn't think about just isolating an event like this.

[Tolls]

Understanding a big project that exists is much the same as designing a big project from scratch.
You need to get an idea of what the thing does as a whole, and then break it down into manageable areas.
In the case of pre-existing code this has already been done, so you can skip the various layers of breaking it down and go straight to "this screen does this, so what exactly does this button do".