I've started reading about the 6502. In this PDF here, what are the internal data bus and internal address bus used for? Are they used to move data between the internal CPU components? or move data between the CPU and the other system devices?
From your PDF, the diagram labelled SY6500 Internal Architecture pretty much nails it. Everything on that diagram is inside the CPU. The arrows labelled AB0 - AB15, D0 - D7 are wires that lead to the chips address and data pins respectively.
Are they used to move data between the internal CPU components?
Yes and no. The internal data bus is used to move data between the components shown to its left and the processor status register to its right. These are all components on the chip. The two address buses (ADL, ADH) are used to move data (data that represents addresses) from internal components to the external address bus.