where can I save my themes & plugins offline?

Hi,I’m brand new here, it’s about an hour! I was wondering how can I save the themes and plugins I want use offline? I’d like to download/upload or(whatever its called) themes and plugins I want to use for different sites in one place.

Is it called that called a local wordpress? And what’s the difference between upload a theme on wpmudev dashboard or ftp file?

PLease help!