This is only a way to backup and restore installed packages on your system.
I use this when I change from workstation to have my trusted set of packages installed 😉
You might need to install dselect as it’s no longer part of a standard Debian install:
apt-get install dselect

And then:
dpkg --get-selections > selections

Install the new Debian system, copy the selections file and then:
apt-cache dumpavail > "$avail"
dpkg --merge-avail "$avail"
rm -f "$avail"
dpkg --set-selections < selections apt-get dselect-upgrade