The back of your phone doesn't really tell you who made it; it only tells you what company put all the pieces together.
Phones, like all other consumer electronics, are a collection of technology from many sources. A lot of it is done in-house — Samsung designs its Galaxy phones and decides how everything is going to fit together then supplies a sort of framework for every piece, both hardware and software. Many of the pieces that make it actually work though, come from someone else and had to be licensed.
