On the outside of the car. Trucks only.
A used tailgate is priced on 82 generations across 27 nameplates, listed below by make and model. Open one for the asking price on that car and what else the part fits.
Hood, hatch and tailgate struts hold the panel up under load. Support the panel before you take a strut off, and bring a helper for anything you cannot hold with one arm.
Typical sizes, not a promise for every car. Everything to bring to a pull-your-own yard.