you can install the sound libraries anywhere you like, that’s no problem at all. You can move them at any time using the Steinberg Library Manager or you can move them yourself and just doubleclick on one library - the rest will be taken care of automatically.
Cubase itself requires disk space of about 1 GB give or take so it doesn’t take up much space.
Hope that helps
BTW: Cubase is fantastic and I am saying this as a user, I’m not associated with Steinberg in any way
Just to add to @Reco29’s excellent info, if by “without ever installing them” you also mean without the download assistant using local drive space you’ll need to change the download location of the SDA as well. So you’ll first change the default download location in SDA, and also change the default library location in the SLM. Then you can download and install without taking any internal space at all.
@Reco29 1 Gb? Wow! 10, 20, 30Gb no problem. But I know I don’t have enough space to install it all and move it afterwards.
Thanks for the info and your enthiastic cheering for Cubase.