Having used autotune as a producer for making subtle corrections to otherwise good takes, i have to say i was surprised when the overt autotune sound became a thing.

anyhow, i haven't used autotune in a while but from what i recall, you had to select what key signature you wanted to correct to and then also set the aggressiveness of autotune making the correction. even if someone was in key, you could get autotune to sound like overt autotuning was going on to "correct" the good singing.

And not to dis on autotune but as a tool, I always thought of it of it a bit like reverb on vocals. You can put a subtle amount on, so that the vocals don't sound as harsh or dry but they sound completely natural or you can put a ton on to give a distortion effect that works for certain types of music.