So... did this kid assemble the whole thing manually brick-by-brick, or did he use some kind of compiler? Does minecraft have macros? Stored procedures? Hard to believe every individual brick was placed by hand.
No macros or compilers per se. There are map editors that allow you to make copies of existing groups of blocks, like replicating seven-segment displays.
There are not really any repositories of existing designs, so the logic was probably all done 'by hand'.
He probably either used Creative Mode (unlimited stuff), peaceful difficulty (no enemies) + a superflat world (don't have to mine things out of your way) or mcedit, which lets you edit map files out-of-game and place whole sections of stuff based on schematics.
Most likely he used an out-of-minecraft editor to put together logic gates that can be copy/pasted, and then larger structures. The map format for minecraft is pretty easy to edit, and there are CAD like programs that'll build out an environment you want.
There is a Lua-based scripting mod but I doubt it is commonly used for things like this. As another commenter mentioned the use of copy-and-paste mods is much more likely.
