Your claim that
If you interrupt the process during the first phase, and start over again, it downloads all packages again.
is completely wrong! Apt is a smart package management system and it never re-downloads packages required to satisfy an installation request twice, not even for a single package or .deb file.
.deb files which are half-downloaded reside in
/var/cache/apt/archives/partial directory and with
ls command (in 16.04, you need to use
sudo ls) you can check those files there.
You can verify this by looking at the
apt-get manpage, where it says about
Storage area for package files in transit. Configuration Item:
Dir::Cache::Archives (partial will be implicitly appended)
You can even pick of one of those files and manually resume download using
In the background, Apt uses
wget program with
-c switch, which from man-page means
-c, --continue resume getting a partially-downloaded file