Hello! Welcome to my article!

Here, I will tell you about some theories that I've made during the time that I've been playing Minecraft!

The End: Endstone and Strongholds

The End seems like a very barren island that can be only accessed through a portal, and endermen are usually seen taking away blocks. This has led me to think that endermen possess the power to not only teleport though the Overworld, but also through dimensions. Strongholds were created by past humans, or Steves to protect themselves from these creatures from another dimension, they found out how to reach it, but weren't able to survive to the Enderdragon. The endstone might have been created by the endermen, they took the blocks they stole to the end where their energy was absorbed by the endcrystal so that they could maintain the Enderdragon alive.

The Nether: Mobs and Castles

The Nether, sometimes called, or at least I've seen that people call it "Hell" or "Underworld", is a... well... hell-like dimension, were normal Overworld mobs do not spawn. My theory behind this, is that it IS the underworld. This is why normal mobs cannot naturally spawn. The Nether is were some mobs go when they die. The Nether portal is created by a reaction of fire to the Overworld's obsidian. As we probably know, obsidian in Minecraft has different properties to our obsidian. First, obsidian in Minecraft can only be mined by diamond, while obsidian in Earth is not that hard at all. So, Overworld's obsidian has different properties, and maybe it reacts with fire to create portals. Now, I think the Nether is the underworld, or hell, because even it's biome is called Hell. Try it, open the debug screen (F3) and look at the b: part, it says Hell. So, which mobs turn into which Nether mobs?
Wither Skeletons: Normal skeletons. Use a spawn egg: skeleton in the nether, it's fun!
Ghasts: Creepers. Both have the ability to make things explode. Both look sad.

Blazes: Skeletons, creepers, humans. Blazes are complex creatures. They might be souls of mobs looking for revenge.

Zombie Pigmen: Villagers, humans, zombies. Villagers were going to be pigmen. You can still find their texture. There's not much explaining behind this.

Nether castles: This theory is much simple. Mobs built them. The end. :P

Well, this is it! Hoped you liked my first blog! If you have any theories, or would like to contradict one of my theories, leave it in the comments! :D