The answer is all three are correct!
The Gang of 500: Many prominent Wikipedians believe most of the content is written by a small core group, a Gang of 500, who mostly know each other and are familiar with Wikipedia rules and protocols. Jimbo Wales, Wikipedia’s founder, has argued this in speeches. He says Wikipedia is written by “a community … a dedicated group of a few hundred volunteers” where “I know all of them and they all know each other”. Really, “it’s much like any traditional organization.”
The Anonymous Horde: The opposing view is that Wikipedia is written by a swarm of anonymous Internet users, each contributing a sentence or a word, with a coherent encyclopedia emerging from the combined result of all these individually small efforts.
Anyone may contribute to Wikipedia. There are editors who review and correct submissions. Lots of people think that what they read, copy and paste for Quizz Club answers are written by "professional authors". This is not always the case. That's why when you use Wiki for your answer, you need to check it for spelling, correctness and grammar. And check other sources to verify what you believe is correct.