It transfers contents of a local file ~/.backpack and itself to remote host, sources it and continues with normal ssh session.
works best as alias ssh=backpack
won't create any files on remote hosts (even temporary)
tries to fallback to normal ssh when remote shell is not bash
self-replication allows you to use backpack again directly from remote host, in this case backpack will keep original local file as you go deaper from host to host."
dbus can also start a program. For example when one notification was generated and no notification daemon is running, then dbus launch one to handle the request.
I think you have confused the apt command with the apt-get command. apt-get doesn't handle files, while apt has it since the very first version. This is one of the important differences between the two commands. This was one of the main reasons why I have been using only apt for years.
They're waiting for Debian developers backporting the patches.