The UI Library
The UI Library, or User Interface controls Library, is the long strip of user interface controls just below the toolbar. It lists all of the UI control types that are included as well as Assets, Icons, and Symbols. The main goal of the UI Library is to let you add UI controls to the canvas, but you can also use it to see what's possible and to get inspiration for your wireframes.
To add a new UI control to the canvas, select the control type you wish to add then either drag it to the canvas below or simply double-click and it will be placed on the canvas for you.
The UI Library can be toggled on and off in several ways: clicking the icon to the right of the Quick Add box, using the keyboard shortcut CTRL/⌘+L, or by selecting View > UI Library from the application menu.