The first Qur'an was written within two years of Muhammads death. The Qur'an was standardized under caliph Uthman several years later, and all other copies were ordered to be burned or buried, or however one would respectfully retire a holy book. The closest English word for Qur'an is generally regarded to be 'recitations', as it was not originally meant to be a text. So, to maintain the integrity of the Suras (chapters, sort of), each one was required to be verified by multiple independent witnesses. Written Arabic was even elaborated to include information about how it was to be read (literally read aloud, not interpreted).

The relative speed with which this all occurred meant that some of these sources were indeed very close to Muhammad during his life. His wife A'i'sha, for example. Still, you can raise doubts by pointing out that the Suras may not have survived oral transmission fully in-tact, or that changes may have occurred over the 20+ year gap between Muhammads death and it's final transcription, and indeed many scholars do. I believe there is some degree of variation in manuscripts and sources.

But the general view among believers is that the Qur'an is the literal, unadulterated word of God, and we know that great care was taken to preserve it.

Not an expert, just took an Intro to the Qur'an course.

I recently started listening to a podcast called Data Over Dogma. They talk about some various similarities and differences in various scriptures/manuscripts. It's pretty neat.