Ohhh, I thought it just made the computer do all the work. Like, they input what it would do and when it would do it to nail perfect timing down or something. But I dunno anything about it, as you can probably tell from what I just described!
nope, it's quite the art in fact, takes a really long time (for some games it can take an hour to the min of final video, but most are shorter)
And beating other people's speed runs also happens. It's an intersting field
Normal "pro" TAS do it frame by frame, and use heavy save stats, although there are other tools, it's worth looking into