Seriously most discussions of metagaming I see are like "I'm running a new campaign and my so-called friend whom I've played D&D with for twenty years BETRAYED ME by having their character use FIRE on a TROLL without first establishing in game how come they could even begin to fathom that a troll might be vulnerable to fire!? How do I punish them for it!?"

Not using fire on a troll until I roll high enough on my "do I know that fire hurts trolls" check sounds like really boring gameplay

Fire hurts troll, magic hurts gargoyle, you know this much simply by virtue of existing in D&D, now armed with this knowledge how do you solve my gargoyle in an anti-magic sphere puzzle?

Like, I'm dumb as shit and where I come from trolls don't even exist and even I know fire hurts trolls? There's no way that Morningwood the Elf who has to deal with trolls being real things that might exist right outside his elven home doesn't know that fire hurts trolls, in spite of him also being dumb as shit!

I think knowledge of fire hurting trolls would work in that world like knowing the fae don’t like iron in ours. There would be so many tales and stories told to children and adults of all the monsters that they know about. There would be hints and clues on what hurts what monster. Dungeons and Dragons isn’t an isekai. Your characters aren’t thrown into the world with absolutely no knowledge. They grew up there. They know things. They had a life before they went adventuring.

Yeah, exactly

And incidentally if you do want to run an isekai (and why wouldn't you isekais rule) don't use D&D if you want your players to have that "having to learn how the rules of world work" thing because your players will know how rules of world work and that constant game of "do i get to use my knowledge" actually sucks

HOWEVER if part of the isekai is "the main characters are D&D nerds who know how D&D work and they're in D&D world now" then run it in D&D and let them use their player knowledge

This.  I was going to say that there’s probably in-universe nursery rhymes about killing trolls with fire and werewolves with silver and such.

I love all these explanations so much and they are so much more valid than mine.

“How did you know fire hurt trolls?!”

“Um. It’s FIRE? Pretty sure FIRE hurts most things?”