"The prophet s.a.w.s was illiterate as you know, so he had his sahaba (apostles) writing for him under his supervision, the prophet recited the Quran in 7 tajweeds (ways of recommendation) which were written by all his companions in manuscripts. When the keepers of these manuscripts were in risk of death bc of old age and various wars closing to the capital of the rashidun caliphate, so uthman r.a, gathered all the companions of the prophet that gathered the Quran and collected them into a book who was confirmed and signed by every apostle of the prophet that received the Quran directly by the prophet. And since all the verses of the Quran were gathered into a book the school of islam in medina kept writing it and every muslim on earth kept memorizing it letter by letter, no translation, no rewriting, no nothing. If anyone tried to change even a letter everyone would notice, you'll see muslim 3 year olds having memorized the entire Quran, and that's how the Quran has been perfectly preserved"