Help Me Understand On-Hit Builds Vs Critical Ones

I should be ashamed, but i'm tired of not understanding the "Why" we do things the way we do here... So, even if it's dumb to ask, I need to actually understand this: 1. What even is on hit building? (rage blade, wits end?) Why is it strong? Why do we go on hit for things like Varus/ Kog Maw but not for Jinx? How do you know which one is better for the ones who do both (like vayne, kaisa?) 2. Why does a pick like kalista bypass both builds and just goes lifesteal?? Is that a thing?
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