There are buses between MK and the Studios and AK and Epcot (as well as between AK and MK and between AK and the Studios), but there are only buses from the parks to Disney Springs after 4pm, and none from Disney Springs to the parks at any time.
Buses run between all of the major theme parks, except between MK & Epcot. To get from MK to Epcot, you take the Express Monorail to the TTC, and then transfer to the Epcot Monorail. There is a bus between DHS & Epcot's main entrance, and there is also a boat between DHS and Epcot's International Gateway entrance.
As PP said, buses run one-way from all of the parks to DS in the evenings only, but there are no buses from DS back to the parks.
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