Every scripture has two authors: a human and a divine. God is fully and completely the author of every word of scripture, but so is each human author of the text. The true humanity of the authors is on full display as each exhibits unique interests and writes in a unique style. The divine authorship of scripture is also on full display, as every word comes with the full authority of God.
According to 2 Timothy 3:16, all scripture is God-breathed or inspired. In other words, inspiration applies to the scriptures themselves, not to the process by which they were produced. The word inspired is a result word, not a process word. The writers were not inspired. The thoughts were not inspired. The various activities in which the biblical authors engaged while preparing to write were not inspired. Scripture itself was God-breathed, and that is what inspiration means.
Nevertheless, God did use a process that resulted in the inspired text. The Bible does not refer to this process as inspiration, but understanding the process helps to explain how the finished product (scripture) could be spoken of as God-breathed. This process is described in 2 Peter 1:20–21.
In the preceding context, Peter discusses the Mount of Transfiguration, where he and other apostles witnessed the splendor of the Lord Jesus and heard the heavenly voice (1:16–18). Having drawn attention to the magnificence of this incident with expressions like majesty, honor, glory, and excellent glory, Peter then pivots to the subject of scripture. His point is that the written scriptures constitute a firm and reliable word from God (1:19). This is the point that he will explain more fully in verses 20–21.
The thrust of Peter’s argument is that the written scripture (or, more specifically, every “prophecy of scripture”) constitutes a revelation from God. God truly spoke on the Mount of Transfiguration, but He speaks just as truly when He reveals Himself in the Bible. This is Peter’s whole point: the Bible is from God. The passage is about where scripture comes from. This emphasis is underlined by the leading verb in verse 20, which is the verb for becoming or coming into being. Peter means to explain how scripture came to be, i.e., how it originated, and he starts by stating how it did not come to be.