This almost belongs in the Addons forum, but anyway:
What the SIX_EditorUpdate does is a simple config hack: it changes a single variable within the config of for instance most buildings (and their ruined counterparts), which makes them usable in the editor. This is the scope that i0n0s talks about (something like scope = 1 or whatever instead of scope = 0). Since these buildings are already configged with default ArmA, no addon dependencies are created and everyone's happy!
The reason I believe, btw, that BIS didn't allow buildings and such to be visible and usable in the editor by default is because once they're visible in the editor their names in-game also change to reflect that : so for instance instead of "All, go to, House, at 5 o'clock!" it now says something like "All, go to, dum_mezo_5, at 5 o'clock". Which is a little uglier than just "House"
Anyway, any 'hidden' objects that SIX does not unlock will require an addon, since they're not configged. They just have a model. Imagine it like you're an addon maker, and you're given a model by a friend. You can put this model into a folder and .pbo it and put it inside your Addons folder, but the model won't show up in game until you've actually written the config for it.
Sadly, the ed.update that I know adds a lot of these (1.02 by someone or other) is a bit flawed, with really stupid naming conventions (all the "addons" are named stuff like AAAAAAAAAAA1), superfluosness (it adds the houses and ruins that can be unlocked with SIX_EU under its own class names, meaning if you have both addons loaded you'll have two sets of everything), a lot of extra, frankly mostly useless crap (like garbage dumps that can be inflamed, a "movable" version of the static weapons that doesn't seem to do anything...) and a bunch of random errors (certain objects have texture errors for instance and will tell you so when you try starting a mission with them). It's a good effort, but someone should really clean it up a bit..
On that subject, Mikero started a very nice little "family" of Editor Upgrades over in the Beta Testing area, but sadly he hasn't gotten as far as "all that extra stuff" yet. Maybe if someone pokes him about it he could continue to create the ultimate, non-cluttered Editor Upgrade
Wolfrug out.