As far as I know, the only direct bus from the settlement area goes to Wilderness Lodge, so this is convenient if you are staying at Wilderness Lodge. And as far as I know, there is no other internal buses other than the Purple, Orange, or Yellow lines that go throughout the Fort Wilderness stops and the front parking lot.
If you are staying at the Contemporary, there is a boat to FW, if you are at Poly or GF you can take the monorail to MK and get a boat from MK to FW, or you can get a boat from CR to FW.
If you are staying at the Epcot Resorts, Moderates or Value resorts, you'd need to get a bus ride to MK and transfer to boat, or get a taxi/uber to the front of FW parking area, then take an internal bus from the front to the back settlement area.
Resort buses go to and from Theme parks, so that is where you would swap buses to go to other resorts. I don't think there are any direct resort to resort buses...? With the few exceptions, like WL/FW or some random times when a bus driver goes from WL/CR/MK, or Poly/GF.