How did the sayings of the prophets and human beings become part of the Qur'an and characterized by miraculousness (i'jaz), despite being their words and not directly the word of Allah?

The Qur’an is truly the Speech of Allah, in both its letters and meanings. From Him it originated, and to Him it shall return, including the stories and statements narrated about the prophets and others. The entire Qur’an is the Speech of Allah. The qasas (stories), with an open ṣād, is the Speech of Allah. As for the maqṣūṣ (that which is narrated), which is the recounted speech, it is the speech of human beings, which Allah mentioned either for us to emulate them or to warn us against their ways. This does not mean that the letters and meanings spoken by others—and recounted by Allah—are exactly the same as those in the Speech of Allah. Rather, those others had letters and meanings that existed in them, and then, when Allah Almighty recounted their speech, letters and meanings that befit Him, عز وجل, also came to exist in His Speech, the specific realities of which nothing shares with Him. Agreement occurred only in the common denominator, which is the meaning conveyed by the word.