How Dan Brown knows everything about the Opus Dei the Illuminati and the Freemasons in his books?

Because as it appears in The Da Vinci Code and Angels and Demons, Mr. Brown is the author.

Let me make this easier. Dan Brown is a writer of fiction, made up stories, books about events that are not entirely real. The reason that he knows "everything about Opus Dei, the Illuminati, and the Freemasons" as they appear in his novels is because he made it up. This is what writers of fiction do, some better, some not as well.

Look at it this way, the reason that John Ronald Reuel Tolkien knows everything about Hobbits, elves, dwarves, orcs, goblins, trolls, the dragon Smaug, Ents, and all the other inhabitants of Middle Earth is because he created them for his stories: The Hobbit, The Fellowship of the Ring, The Two Towers, and The Return of the King, as well as his other works. The same is true for Dan Brown.