Buildings and decorations can be selected from the Buildings/Elements panel. To access it, please tap on the round divided button in the bottom left corner of the screen.
A panel will open where you can choose the type of building or decor you'd like. Place the building on the ground and tap the button with the green check mark. To start the construction, you need to assign a settler to it. You can do it by tapping on the building and selecting the settler from the roster.
Wait until the construction is finished, your building/decor is ready to use!