This article should give you the insights you need:
An important take-away is that since park hopping doesn’t begin until 2:00 PM, the park-to-park bus lines have a delayed start.
You can use the interactive transportation map here do find out what locations are interconnected, but it does not address start/end times. Buses are not always the most efficient way to get between parks.
And here are a few statements from Disney:
Looking for information on how to get from one park to another? Download the My Disney Experience mobile app to view routes and stop locations for various modes of transportation—including buses, monorails and water taxis.
Please tap “Get Directions” in the My Disney Experience app or ask a Cast Member to assist you in planning your trip from one park to another. The most direct route may involve a combination of bus, monorail, boat or walking.