You make them out of wood or smooth stone (not cobble). The wood ones can detect items and mobs/players, the smooth stone only the mobs/players. You make them like this:
xxx
xxx
mmx
where x is nothing
and m is wood or smoothstone
Chat with our AI personalities
You use redstone things like levers/buttons/pressure plates.
You could build a cool building, it is also a crafting material for buttons, stone pressure plates, stone slabs, stone bricks, and redstone repeaters.
Yes, as well as animals, people/players and sometimes other objects if they glitch.
Yes and no - Iron Doors can't be opened normally, they can only be opened when supplied by charge, which can be provided by levers or buttons, but also by redstone circuitry. Villagers can activate pressure plates, so if you wanted to add Iron Doors to a village, you could set up a system of pressure plates and redstone repeaters to make a door stay open for a few seconds once the plate activates.
I didn't make Minecraft.