I don't even know mainstream narratives because I know it's all garbage. What were they told?

Story we got told in school is because he was just expansionist and he had gotten away with everything up until that point so why not. Honestly I'd say that's at least partially true. He wanted to at least restore Germany to it's greatest territorial extent. He likely had no idea invading Poland would finally provoke the allies to declare war, since he wasn't aware of their secret treaty and they'd mostly been friendly with him up until then.

Germany attacked Poland in Danzig because jewish poles were killing German's, correct?

And because Polish troops crossed the border for funsies and attacked German border outposts firsts. Except that turned out to be a false flag by the Germans and I'm having trouble finding evidence for the claims about Germans in Danzig being attacked that don't come from the NSDAP.

Plus, Germany had already divided up Poland with the Soviets at that point, so the attack was clearly premeditated, not based on some humanitarian emergency.

If we're talking about objective reality, history is usually more complex than "good guys vs. bad guys."

The reason we talk in those terms is because our emotions are geared towards simple ingroup vs. outgroup calculations. The primary purpose of politics and culture is to determine the outgroup and defend the ingroup, hence Plato's assertion that: "Those who tell the stories rule society."

In the case of modern germany they call Hitler "the bad guy" because those who tell the stories have taught them that they are their own enemies. Hopefully that changes, but either way the only way for Hitler (an invader) to become "the good guy" in Poland is for them to be taught the same lesson, which should not happen and I hope does not happen.