Compare

Two tools that overlap, what differs between them, and which one to use.

apt upgrade vs full-upgrade vs dist-upgrade

apt-get upgrade installs nothing new, apt upgrade installs but never removes, and full-upgrade does both. dist-upgrade is the same command under an older name.
apt vs apt-get

The same package ships both, and they do the same job. What differs is who each is written for, and only one of them is safe in a script.
cron vs systemd timers

Debian ships both and uses both. What differs is where the output goes, what happens to a missed run, and whether you can test the schedule.
Hard links vs symbolic links

A hard link is another name for the same data, a symlink a small file holding a path. What that means when the original goes, when sed edits it, and on disk.
remove vs purge vs autoremove

Not three strengths of one operation. Two of them differ over configuration files; the third acts only on packages you never asked for.
sh vs bash vs dash

On Debian /bin/sh is dash, not bash. That one fact explains most scripts that work at your prompt and fail the moment something else runs them.
ss vs netstat

netstat is not installed on a Debian system and ss is. The flag translations, the filter syntax netstat cannot express, and when the old name is still fine.
su vs sudo -i vs sudo -s

sudo -i, sudo -s, su and su - leave you as root with different directories and audit trails. Which to type, and why su fails on a stock Debian install.
which vs type vs command -v

which searches PATH and nothing else, so it reports that cd does not exist. What type and command -v answer instead, and which one belongs in a script.