Our experience went like this:
Referral - school and us completed forms describing DS
Appointment one:
In person, with DS, met with consultant paediatrician. He took developmental / medical history, and DS got weighed and measured. He agreed in this appointment that it needed further assessment and referred to MDT (SALT / Educational Pysch).
Appointment two:
Phone appointment, DS not present, questions mostly about sensory issues.
Appointment three:
ADOS, I waited in the waiting room, DS went in and played games / answered questions about his likes / dislikes / feelings / friendships. If he had wanted, I could have sat in the room with him, but he wasn't fussed.
Outcome:
Given over the phone in the afternoon after appointment three. If they hadn't been sure at that appointment, they could have done a school visit, but it wasn't needed.
I hope it all goes well for you tomorrow OP!