If you are talking about how they make the functions the mods do themselves, then that is a matter of many, many combines commands. Furniture most likely is from multiple invisible armor stands with blocks on their head. If you put 20/30/40 armorstands in the same place with different block on their head and different head/body orientations, you can basically make a block model without resource packs or actual mods.
Also, the way these creations work is most likely on a fill clock that is constantly powering a set of command (20 times a second) that check for you placing down a block or something, and doing stuff when that action occurs (like spawning in all those armor stands when you place down a certain spawn egg or block that creates the furniture).
But if you are talking about how they put all that stuff into one, two or three commands, then that is call a one-click command block structure spawner. These, when pasted into a command block on any world and powered, will replicate the command block structure the original creators made.
While there are many mcedit filters out there that will do this, the one that will copy the blocks in raw (not modify the structure or add anything) is GentleGiantJGC's mcedit filter.
If you would like to get started in making these types of things, i suggest looking through the Gamemode 4 modules. Gamemode 4 is a way to play MC with many of these modules installed in the world. They already have 35 modules, with things from undead players to extra hearts with heart canisters to using minecarts up and down walls. The most notable module is the custom crafter which is the universal way to craft special items that lots of other modules use to do their own special things and how to create a custom crafter recipe.
You can use command blocks. Place a redstone torch under the block you want to break and a command block above. Then, the command is:
/setblock ~ ~-1 ~ minecraft:air 0 destroy
Other than that, there is no vanilla way. (this is also not very vanilla)
To pick it up by a hopper place the torch on a wall under it and place the hopper at least 2 blocks under the cobble to make sure that you don't power the hopper
Best Answer
This page has a paragraph specific to the power output through a comparator on a furnace.
It's pretty confusing, though.
Basically, for the full 15 signal you need something in all 3 slots of the furnace. If you're feeding in smeltables and fuel (assuming they are in the right places, and full), then anything above 10 would mean that there is something that has completed smelting.
That is probably over-simplified, but I hope that helps.
To further attempt to answer your question; The contents are summed and the resulting value of all contents are output through the comparator.