how can I move scanned photos from the Desktop folder to the Library?

The easiest way may be to select all the pictures you want to move and then drag them to the Photos icon on the dock. Photos will open and show you the Import page. If you have your pictures in folders, then Photos asks if you want to Keep Folder Organization. If you say yes, the Mac folders of pictures will go into albums with the same name (which, weirdly, are inside of folders of that same name.)

Another option is to choose File>Import and follow the dialog to locate the photos you want to import.