Sonnet includes WPBakery Page Builder by wpbakery. You can use WPBakery Page Builder inside Sonnet for free. Please note that it is not a freeware though. You can not use it apart from Sonnet unless you purchase the appropriate license.
This documentation on using WPBakery Page Builder is the very basic information you need to get started. Please take a look at the important links below that have additional information about using WPBakery Page Builder.