All communication between the individual major components is via the system bus. The bus is merely a cable which is capable of carrying signals representing data from one place to another. The bus within a particular individual computer may be specific to that computer or may (increasingly) be an industry-standard bus. If it is an industry standard bus then there are advantages in that it may be easy to upgrade the computer by buying a component from an independent manufacturer which can plug directly into the system bus. For example most modern Personal Computers use the PCI bus.
When data must be sent from memory to a printer then it will be sent via the system bus. The control signals that are necessary to access memory and to activate the printer are also sent by the CPU via the system bus.