This is what I have compiled from a variety of sources. It's 2019 specific, of course.
Fiesta Bowl: CFP vs. CFP
Peach Bowl: CFP vs. CFP
Sugar Bowl: Big 12 Champion vs. SEC Champion (if neither are selected for the CFP)
Rose Bowl: Big Ten vs. Pac-12
Orange Bowl: ACC Champion vs. Notre Dame/SEC/Big Ten
Cotton Bowl: Highest G5 champion vs. at-large