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I suggest you have a look at the source code and then ask more specific questions based on what you discover. There are fortunately many people on ROS Answers who are willing to help others, but very few have the time to write a comprehensive answer to a extremely broad questions like yours.

You're much more likely to get answers to a question like "I have looked into package X's code and do not quite understand what is done in function Y, can someone help me?" than to one like "Please explain the internal workings of package X in detail" (such as your current question above).