It's not a "patch" in that sense of the word, it's called a repair doubler, which means a repair consists of two "patches", one on the inside and one on the outside, fitted well around the damaged area.
If your plane's cabin gets hit but a truck, the damaged part is first sawn out to prevent growth of fatigue cracks and a repair is crafted to match the construction of the aircraft body precisely, including the internal stringers as far as they are concerned. These bits are fitted to the aircraft using double rows of rivets and in case of wings' torsion boxes and pressurized cabins have to meet very strict rules about how large they have to be, how many rivets have to be used, how far they are apart, etc.
