As Idran says, "canon" is just a shorthand way of saying "the media that comes from the source." It doesn't mean "facts" or "truth." It just means it's the stuff made up by the creators or owners of the property rather than the stuff made up by other people. And since it is all just made up, a canon can contradict its own "facts" quite a lot, with retcons or mistakes or whatever. So it's not about the "factual" nature of any single piece of information. It's just a way of categorizing where the overall story comes from.
Nobody "determines" canon. It's not an official seal of approval. It's not a label that some guy at CBS or Paramount goes around stamping onto things. It's a term of criticism. It's a description, a way that people talking about a fictional series refer to the original work as a way of differentiating it from derivative works like fanfiction, tie-ins, or pastiches. Nothing needs to be declared canonical; if it comes from the creators or owners, canon is simply what it is, by definition.
It's like, say, calling something a hill vs. calling it a mountain. Calling it that doesn't make it what it is; it's just a way of describing and classifying what it is.