Libraries are all different. At the first one I ever used, they only had one desk. You borrowed and returned books at the same place. It was the same at my local library in Madrid. However, the new, modern library in my home town has a desk where you borrow books but when you take them back, you scan the barcode at a small machine and put the book on a shelf. You don't have to see/speak to a member of staff at all to return your books.