Generally speaking, BW shares buses with the S/D. YC/BC pretty much have their own buses. BUT.....there are some times when they do share a route. Usually mid-day they all combine routes. But for the busier times of day, park opening and closings, BW/S/D are one route and YC/BC are another.
If they do share a route, the bus tends to go to BW, S, D, YC, BC. The route 'could' start with YC though...but my experience has shown BW is usually first. But again, nothing is etched in stone with the buses at WDW.