There were a couple of BCS years where all 4 BCS games were open to anyone, if the anchor teams were in the BCS championship.

Now there are 3 completely full NY6 games (Sugar is always SEC v. B12, Orange is always SEC/B10/ND v. ACC, Rose is always P12 v. B10) and 3 completely open ones (Cotton, Peach, Fiesta).

That's why 2019 is a crappy year for BYU to go undefeated, because Peach and Fiesta are in the Playoff, and the top G5 champion is guaranteed one of the Cotton Bowl spots, so if BYU goes undefeated this year (and doesn't make the playoff), their reward would be playing Boise or UCF or another G5 champion in the Cotton Bowl.