Migration script

Hi people, I'm new here and also new to the GNU/Linux world though I've been playing around with some distros lately, especially debian based, and I'd love to learn more!

I got a question about the Migration from Ubuntu to Trisquel Script.(https://trisquel.info/en/wiki/migrate-ubuntu-trisquel-without-reinstalling)

As I'm new I don't really understand how the script works. I'd like to modify what packages are removed and installed, but if it isn't possible I'd still like to learn how the whole script thing works. Till now I understand that the mirror, release and edition variables are to allocate the needed package repository for the specified edition of the release through that mirror.

Also I acknowledge how the last part works where the remaining non-free software on the system is removed with the user's permission (the Y/n part) and also the part where to install the trisquel default desktop layout.

But what does really happen in between those 2 parts of the script? where a bunch of commands are seen executed for the most part of the migration? and is it possible to modify that part?

cat << EOF > /etc/apt/preferences.d/pinning
Package: *
Pin: release o=Trisquel
Pin-Priority: 1001
EOF

"
export DEBIAN_FRONTEND=noninteractive
apt-get update 2>&1 | COLUMNS=500 tee /var/log/trisquelize.log
apt-get -y --force-yes install trisquel-keyring 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-key add /var/lib/apt/keyrings/trisquel-archive-keyring.gpg 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-get remove --purge plymouth-theme-ubuntu 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-get remove --purge plymouth-theme-ubuntu-text 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-get -y --force-yes dist-upgrade 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-get install -y --force-yes --no-install-recommends $EDITION $EDITION-recommended 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
apt-get install -y --force-yes --no-install-recommends linux-image-generic 2>&1 | COLUMNS=500 tee -a /var/log/trisquelize.log
rm /etc/apt/preferences.d/pinning
"

Thanks in advance!