@KittyMcKitty - it will vary from university to university.
After we get the exam results on the Friday, where I work, the central admissions team spend the weekend working out who has met the terms of their original offer.
Then, over Monday and Tuesday, the admissions teams in the departments will look at near misses and decide whether to accept. We make a point of revisiting each application once more, comparing actual and predicted grades, checking for any mit circs that may have occurred since we made the offer, and looking for other quals / relevant experience which may top up a slightly slipped grade. For example, we don't include EPQs in any offer we make, but if someone has an epq in a relevant topic, then that's a bonus.
It's very difficult to call it at the moment: we don't know if grades will be pitched higher or lower than a normal pre-covid year. Either of those can have an impact on the number being accepted by their firm choice or, conversely, dropping to their insurance. This is particularly relevant to depts like mine (sciences in RG uni) where specific A level subjects are mandatory.
What I would say though is not to worry too much yet. Even if your DD slips by a grade, she may still get her firm choice. Of course, I can't say that with 100% certainty, but I see the grades other universities accept for applicants holding them as firm choice, where we're the insurance choice. I can tell you for sure that slippage of one grade, for eg, is frequently acceptable.
(Sorry, that was long!)