Welcome to the forum, pacotelic. We're glad to have you here.
I started not to reply to this thread, but I didn't think anyone else would so here goes.
First of all, I did not develop any of the code for phpdig, so I'm not the world's most knowledgeable person regarding the inner workings of the code. However, I can tell you from having taken a cursory look at just how phpdig stores data in its index that it stores lookup data for individual URLs in the tables called spider, keywords and engine. If you browse those tables, you can get a sense of what most of the data is and what it's used for.
If you want an in-depth look at how they work together, you will probably want to look at admin/robot_functions.php, as that is where index data is written to. There's also a brief overview of how phpdig indexes
here in the documentation.
Hope this helps.