good info... I have added a little more understanding via the linked articles.
Kado's second attached image appears to address my base question (not how to use libraries, but what is going on under the hood - the nuts and bolts of what Windows is doing to provide the view in the user interface). The explanation in the image which says, "When you include a folder in a library, the files appear in the library, but continue to be stored in their original location." seems to address the question. I f I am reading between the lines right, libraries provide a view of another memory location without duplicating the data in a different location. This would make the library view of the library a collection of pointers (in programming language) to the actual memory location where the files are stored.
Did I get that right?
There is a reason for my line of questioning here. I have a friend who has a 500GB HDD with less than 10% free space. She came to me for help because the system crashes when she tries to manipulate large files, and she cannot defrag due to lack of unused space. Likewise she cannot back up the HDD. Quite a few of the large files (some of them over 3GB in size) are duplicated in several folders because she does not know how to use libraries. Instead she uses the type of file/folder organization from the days before libraries were introduced. I recommended copying the bulk of the folders/files to an external drive, but she wants to keep everything on the internal drive.
If my analysis of how libraries work, as presented in this thread is sound, I believe I can help her free up some HDD space. My tentative plan is to guide her through a process to copy enought folders to an external drive to begin the task of creating libraries and to systematically build more libraries to preserve the type of organization she finds to be useful for her purposes. I believe that if we leave the default libraires (documents, pictures, videos, etc.) intact, we can build the custom libraries. Once she has the concept down, I will turn here loose to complete the process on her own.
Now... Using the perspective of the full HDD and my analysis of how libraries work under the hood, is my reasoning sound enough to attempt to proceed with the project?