Panel 1. The PC might know about trolls' weakness to fire. They might not. Has that character encountered or heard about trolls' weakness to fire? First-level character on their first adventure? Not likely. Tenth-level character on their 100th adventure? Yeah, quite likely.
Panel 2. The PC is speaking in game mechanics. The characters don't know the game's mechanics. They don't think or speak in terms of armor class or hit points. That's pure metagaming. I'll give you that it's likely the player telling the DM but inserted into an in-fiction panel. This is the closest to a maybe not metagaming.
Panel 3. People turning into zombies from zombie bites is not how D&D works. It's how zombie movies work. So the player is using their knowledge of zombie movies to dictate their actions in the game...when zombies don't work like that in the game world. So again, metagaming.
Panel 4. Again, the player is taking their out-of-game knowledge of how they think megadungeons work and making decisions for their character based on that. The character cannot know how megadungeons work nor with any degree of certainty that it's impossible for a seemingly human child to be alive on any given level of any given dungeon.
Maybe we should have a separate thread for people to argue about the memes.