Once you installed each package, you can delete the stuff from the download location, because indeed, the installer unpacked everything and copied all that 8GB somewhere else (probably now onto your C drive).
But you don’t move stuff into the Library Manager, but the Library Manager only keeps track of where all the libraries are and makes them visible to the instrument plug-ins. And the Library Manager can move the libraries easily from the C drive to another drive, thus freeing space on your C drive.