You can review an appointment after it has happened. A prompt appears in your bookings, and the studio may also send you a link.
Reviews are tied to real completed appointments, which is why you cannot review a professional you have never booked. It is also why the ratings on this site are worth reading.
A studio can respond to a review publicly but cannot delete one because they disagree with it. If a review breaks the community guidelines — abuse, personal information, something about a different business entirely — report it and we will look at it.