Looking at each team's top 12 games by viewership (so top 3 per season on average) over the last 4 seasons (where I have data)

BYU
All 12 of BYU's top games are against P5 opponents or bowl games, none against G5 opponents in-season.
1.21M (Vs CCU) is BYU's highest rated non-P5, non-bowl showing.

CINCINNATI
6 of Cincy's top 12 games were against P5 or in bowl games, while 6 were against G5 opponents in-season.
3.12 M (Vs UCF) is the highest rated non-P5, non-bowl showing.
2.88 M (vs Memphis) is the highest rated non-P5, non-bowl, non-UCF showing
Cincy has 6 non-bowl G5 games (UCF, Tulsa, Houston, Memphis, Memphis, UCF) with more viewers than BYU's best

MEMPHIS
5 of Memphis' top 12 games were against P5 or in bowl games, while 7 were against G5 opponents in-season.
3.39 M (Vs UCF) is the highest rated non-P5, non-bowl showing
2.95 M (vs SMU) is the highest rated non-P5, non-bowl, non-UCF showing
Memphis has 5 non-bowl G5 games (UCF, Cincy, Cincy, Houston, SMU, UCF) with more viewers than BYU's best

UCF
5 of UCF's top 12 games were against P5 or in bowl games, while 7 were against G5 opponents in-season.
4.7 M (Vs USF) is the highest rated non-P5, non-bowl showing
UCF had 11 non-bowl G5 games (USF, Memphis, FAU, Temple, Cincy, USF, Memphis, Cincy, USF, Memphis, Cincy) with equal or more viewers than BYU's best

TL:DR - If any of the schools above are piggybacking off of popular P5-type opponents to bump up their viewership numbers, the data suggests that it is actually BYU