How do you gain an understanding of other peoples source code so you can use their programming techniques yourself? [closed]

  analysis, c++, python, reverse-engineering

for example what are your techniques for breaking down a C/C++ program like:
(obviously this is a complex program just an example)

What are your usual techniques for understanding open source software, extracting useful samples of code and understand them to a point you can use them in your own projects.

I’m sorry if this is a bad question please reserve critiques in the comments/replies below my question instead of as an answer, thanks :).

I am asking this as I’d like to decompose complex projects to use some of their solutions in my own projects so I understand what they do and I can create better solutions myself.

Source: Windows Questions C++