By reading the message and using basic comprehension. If you don't know what something is in an error message then google it!

Far be it from me to defend "I did what the internet told me to do!" but nothing in sudo apt-get install steam would lead you to believe you were about to nuke core system packages. That was a big fuckup for PopOS.

There's also no reason to believe that apt update would be a preemptive solution to that problem, when it hadn't even been reported to PopOS yet. Let alone expect newcomers to Linux, who are just following widely available tutorials, to know that command and what it does.