How does Minecraft make such big maps?

I find Minecraft interesting because its all made out of blocks so most of it is easy to understand how it was made.

How has he made the terrain so large, they couldn't have laid it out by them selves because its way too big.

How did they make the underground caverns so big? I went it in one cavern and it had large waterfalls laid out so they looked good and linked up so how could they randomly make them without them looking out of place?

It was done with perlin noise, or more precisely a creative combination of multiple perlin noise.

Here's a video that explains it quite well.