4.Which structure has the most OP loot (overworld)?
Desert temple
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Ancient city
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Underwater Ruins
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It's OBVIOUSLY the desert well!
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They contain level 50 enchanted leggings, somewhat common notch apples, enchante books, enchanted hoes, etc.
5.Which structure has the most OP loot (nether, end)?
Nether Fortress
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Bastion Remnants
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End cities
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Bastion remnants contain gold blocks, diamond armour, netherite scraps, ingots, and ancient debris, and golden apples and carrots. I suppose it depends on your opinions, leave it in the comments.
6.Which structures contain the allay?
Pillager outpost, ancient city
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Pillager outpost, woodland mansion
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Woodland mansion, desert temple
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village(any type), end city cage
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They spawn in the cages of outposts and jail cells of woodland mansions. (end city cage doesn't exist, but it would be nice, Mojang)
7.Which structure is the largest?
Bastion remnants
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obsidian wall
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woodland mansion
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Ancient city
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Obsidian walls stretched from certain coordinates to the edge of the world, though I guess the far lands could be a candidate for a structure.
8.How many types of structures can spawn in the nether?