The two tabs in your Inventory serve for special purposes:
1. Inventory.
This is used for storing all the items for your quests, additional resources, and bonus chests. Your Inventory's capacity is not limited. All items that you find or obtain will be automatically placed into your Inventory.
2. Warehouse.
This tab is used for buildings only. To place an object in the Warehouse, tap on the button with the picture of a hand, select the icon with a box, and choose the necessary building. At first, there are 2 free cells in this tab, and you can open additional ones with gems if you need any.