Buses are dispatched from 6 or 7 "hub" regions (which coincide roughly with the major theme parks and Downtown Disney) to the resorts, TTC and smaller parks they connect.
There are fixed routes (pretty much, although they will sometimes consolidate and split destinations depending on traffic demand) but no fixed schedule.
Rather, the goal is that a bus arrives at a given stop going to a given (set of) destination(s) at least every 20 minutes -- more often when there is lots of volume.
For example, 1/2 hour before park opening, there will be frequent bus service from the (separate!) All Star and Pop Century resorts to all the parks. More often than 20 minutes, since the volume justifies it.
During the middle of the day, the route to the Animal Kingdom and Blizzard Beach will be consolidated. Similarly, the same bus route may serve Wilderness, Contemporary, Poly and Grand Floridian, whereas later when the parks close, separate buses will go to the WLodge-Contemporary and the Poly-Grand Floridian.
The goal is to provide a minimium level of service all the time while tailoring capacity to dynamic demand.