The playoff bowls will be based on a combo of historical bowl agreements and proximity. Out of the 4 quarterfinal bowls, only the Rose and Sugar bowl have conference tie ins. The Peach and Fiesta will just be based on proximity (not on some historical agreement with the Big 12)
It’s like 95% (or more?) likely that the SEC champ is ranked ahead of the Big 12 champ and none of this matters. But the CFP committee’s website says (a bit indirectly) it will be the highest ranked of the SEC and Big 12 assigned to the Sugar.
For that reason, CBS’ Sunday bowl projection has BYU as a #2 seed in the Sugar Bowl, and SEC champ Alabama in the Peach Bowl. Leaving Miami to the Fiesta Bowl