I think the traditional islamic narrative is that muhammed received the quran in a cave from the angel gabriel, and the fact that muhammed was illiterate is used as evidence that it came from god since it couldn't have been written by him.

I would expect there isn't much certainty on how the quran was written, but do historians have speculations on how it could've been written?

How plausible is it that a succesfull merchant in arabia such as muhammed was illiterate? is there reason to doubt his illiteracy?

Are there any literate individuals that could've helped/participated in the writing of the quran?

I've heard that the quran is a very impressive text in terms of its prose in the original arabic. Would it take a skilled writer to have written the quran? how hard were those to come by in arabia? I've heard that there was a literary/poetic tradition in arabia, presumably that would provide for a decent supply to skilled writers to have written the quran?