4. Which structure has the most OP loot (overworld)?
They contain level 50 enchanted leggings, somewhat common notch apples, enchante books, enchanted hoes, etc.
Desert temple
Ancient city
Underwater Ruins
It's OBVIOUSLY the desert well!
5. Which structure has the most OP loot (nether, end)?
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.
Nether Fortress
Bastion Remnants
End cities
6. Which structures contain the allay?
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)
Pillager outpost, ancient city
Pillager outpost, woodland mansion
Woodland mansion, desert temple
village(any type), end city cage
7. Which structure is the largest?
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.
Bastion remnants
obsidian wall
woodland mansion
Ancient city
8. How many types of structures can spawn in the nether?