In your inventory you will find certain items that you obtain through quests, Great Buildings or special events.
On special occasions, like at the end of a special event, you might receive a special building. That will not appear in your building menu, but is rather placed into your inventory. If you want to place on of those buildings into your city, simply click on the item in your inventory and put it wherever you want - no further costs need to be paid, although a population cost might be applicable. Also note that when you sell one of those special buildings it will be permanently gone from your city and is not put back into your inventory.
Forge Point Packages
When you are among the top five contributors when a Great Building of another player levels up, at times you will be rewarded with some Forge Points. Also, certain quests might give you Forge Points as a reward. These Forge Points are put into your inventory as Packages of 2, 5 or 10 points. If you want to use one of them, simply click it and confirm - the Forge Points will be added to you immediately.
All blueprints that you obtained are also put into your inventory. Here you can see the progress on the collections for the different Great Buildings and enter the construction menu. There you can buy missing blueprints with Diamonds, trade in duplicate blueprints and pay the goods costs to finally construct your Great Building once you complete a collection.