JConte, Norbert, oriole_fan, and sebel all contended that every single play is simulated one at a time, and independent of each other. Results are, they claimed, based solely on the play called, the personnel on the field, and a series of "checks" that determine how the play is proceeding.
I haven't played the game in several years, but it never really felt like that to me. It always seemed like every player - especially on defense - acted more as a part of a group rather than an individual player.
There is another football game online called Goal Line Blitz. It's nothing like GD, it's more like a "football player RPG". But the one cool thing about it is, when you review the play-by-play, you get an X and O representation of every player and what they are doing on that play. It's one of the many things I wish GD would implement, but it's certainly not feasible given the current circumstances.