I think you should see "Get Books" as a search and comparison tool.
It helps you to search for a book in many different online shops, at the same time. You can then compare the price and see if the book has some type of DRM.
But to actually buy and download the book I think you have to go to one of the stores, and download it from there. Just double click the book you want to buy in the "Get Books" window, and you will be taken to that store. Then you may have to register there and so on, just as any online purchase.
Some books are free, and you may double click them and can then download them.
Perhaps Calibre sometimes can figure out how to download for you, from a specific site, and then the meny option to Download will be active. But I have never seen that yet...
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