In the mornings (pre 11am), each All-Star typically gets its own bus for each theme park... as demand warrants.
Post 11am, theme parks convert to a hub central dispatch, and DAK, Epcot and Studios combine the All-Stars. MK will usually always have them split.
As for which resort gets serviced first... Sports gets to be the first stop more times than not... HOWEVER... there are a lot of busses that work backwards... particularly during the busy times. So Movies also gets its shot at being first.