If I understand correctly, US domestic carriers’ flights are repeated daily. Thus if flight 123 goes from airport X to Y to Z on, say, Monday, then Tuesday, flight 123 of the same carrier will also go from X to Y to Z.
But – what happens if the X->Y leg starts at, say, 23:00, and the Y->Z leg starts at 02:00 the next day? Is the same flight number used? Is it then the case that flight 123 goes, every day, from Y to Z and later in the day from X to Y?
Note: Let’s assume no timezone changes to make things simpler.