What is the fundamental common sense in PC Hardware?

Hi, Iam mohammed, and as a starter in tech field i wonder what are the common sense principles or shared knowledge in PC Hardware.
About:
-PC parts
– based on what we measure if a pc part(CPU, GPU,…) was good or not?
-what are the common red flags of a pc?
-what are the mistakes that i should avoid when building/fixing a PC?
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