Becasue of a few reasons that are probably not the best reasons, I need to use an older system (DietPi_RPi-ARMv6-Buster.img) and disable the auto update the system does on boot, is this possible?
Edit: I am already able to use it and I even installed some packages and did some other stuff, but I still need to interrupt the apt upgrade that tries to happen on login. so the question would be, is that done via systemd? a cron? rc.local?
Welcome to our community.
Did you already finished first run setup? Because that’s the only point where an apt package update is forced. During run time, there is no automatic apt package update done. The only thing we do is to perform an apt update to be able to inform user about availability package updates. Ok there is one more package update forced. This is during a manual triggered dietpi-update.
Did you already finished first run setup?
I just did and I think I see what you mean, even after seeing all those apt update/upgrade in the console, after it’s done, it still says “Buster” when I type cat /etc/os-realease 
So I’ll continue then, thanks!
The Debian Buster version will not change, unless you manually adjust source files to point to Bullseye 