They are random and unknown, unless you use the Marketplace to look at the top two (returning them in any order).
Six special buildings are drawn randomly from the deck of special buildings (without looking at them) and placed in a face down pile for use during the Full Game.
However, at most five of the buildings will appear during the game (only three if you are playing the solo full game).
As mentioned, using the Marketplace will let you look at the top two cards in the stack and rearrange their order, which is why you place six cards and not just five.
(You do not use special buildings at all during the shortened game)