Once special buildings are unlocked, they will go into the available pool of buildings that are randomly picked from when a lot is built of the matching type. When building on a lot you cannot choose which specific building you will get, though you can rebuild a lot instantly using bux until you get the one you want. Or, you can tap the main Build button and it will rebuild to a random building as it cycles through all of your lots. Once a special building is built (either instantly or automatically by the game) it will be marked as historical. This mark makes it so the same specific building is always build on that lot until you unmark it as historical. You can manually mark any lot as historical to keep it locked to the same building. This is helpful for missions requiring specific buildings to be build or if you want to groom your city to look a certain way.