There's never just one key.
Talking heads like to try to boil it down to one, or three, or five "keys to the game" because it gives them talking points. It allows them to put some framework on the play-by-play, give it some extra drama.
But it's never about just five things, either.
It's a game with an oddly-shaped ball that bounces funny, played in all weather (except God forbid there is lightning sighted within 10 miles of the stadium), by 11 guys in one color jersey against 11 guys in another color jersey, plus roughly the same number of zebras trying to keep things mostly within the rule book. It's line-versus-line, receivers-versus-backs, QBs and coaches and passing games and running games and penalties and a kicking game and clock management.
There are literally dozens of variables. And every one of them count. Any one of them can be THE key to a particular play. And each of them WILL be the key to a play at some point in a sixty-minute match.
Asking to boil it down to a single key for the entire game is like asking Mozart which note was his most important one. Or asking Da Vinci which pigment made him a success.
Good luck trying to get it sorted. If you just need to have the structure, go with Neyland's Maxims (there were really something like 38 of them, and each one is actually three, four, five or more "keys" hidden within a single phrase, like "ball, oskie, cover, block, cut and slice, pursue and gang tackle, for this is the winning edge." "uhh, which is the winning edge, coach? ball or oskie or cover or block or pursue...I'm confused.").
Go Vols!