You’re wondering what gets replaced during a turbo back exhaust upgrade or replacement. In this type of job, everything between the tailpipe and the final outlet of the turbo is replaced. This is similar to a “cat back” or catalytic converter back upgrade, except the turbo back job replaces more components. A good picture where you can see the difference between cat back and turbo back can be found here. A turbo back exhaust upgrade increases horsepower output by increasing the scavenging action of the gases flowing out of the turbo, usually by increasing the diameter of the pipes.
For more info, see How Exhaust Systems Add Power