The need is: for any given package, download that package and all dependencies, regardless of their installed status on the current system, so that they can be put on some external storage and taken to the other system.
[1] http://rickysarraf.github.io/apt-offline/ [2] https://www.ostechnix.com/fully-update-upgrade-offline-debia...
We're pretty much at the point where you could mirror the entire archive including every arch onto a portable hard drive, the total size is 2453GB: https://www.debian.org/mirror/size