From Wikipedia:

The New Year's Six (NY6) bowls are the top six major NCAA Division I Football Bowl Subdivision bowl games: the Rose Bowl, Sugar Bowl, Orange Bowl, Cotton Bowl, Peach Bowl, and Fiesta Bowl. The New Year's Six represent six of the ten oldest bowl games currently played at the FBS level. These six top-tier bowl games rotate the hosting of the two College Football Playoff (CFP) semifinal games, which determine the teams that play in the final College Football Playoff National Championship game.[1] The rotation is set on a three-year cycle with the following pairings: Rose/Sugar, Orange/Cotton, and Fiesta/Peach.
Using the final CFP rankings, the selection committee seeds and pairs the top four teams and determines the participants for the other four non-playoff New Year's Six bowls that are not hosting the semifinals that year. These four non-playoff bowls are also referred to as the Selection Committee bowl games. These six games focus on the top 12 teams in the rankings; to date during the College Football Playoff era (2014 through 2019 seasons), of the 72 teams to play in a New Year's Six game, only eight have been ranked lower than 12th.
So, in all, twelve schools are selected for these major, top tier bowls. These are required to include the champions of the "Power Five" conferences (ACC, Big Ten, Big 12, Pac-12, and SEC). In addition, the highest-ranked champion from the "Group of Five" conferences (The American, Conference USA, MAC, Mountain West, and Sun Belt) is guaranteed a berth if the group's top team is not in the playoff.[2]