The Hunger Games: The Ballad of Songbirds and Snakes provided more background information about countless concepts introduced in The Hunger Games trilogy. It elaborated immensely on the history of the Games themselves - and many of the ideas that were synonymous with the annual event by the time the original books took place. It also revealed that many ideas were pioneered by Snow in an attempt to save his beloved tribute Lucy Gray from certain death.
Source：https://www.cbr.com/hunger-games-hanging-tree-explained/
