John D. W. Watts gives the basic premise for the multiple-Isaiah hypothesis.
J. D. W. Watts, Isaiah 1-33, rev. ed. (Word Biblcial Commentary vol. 24; Grand Rapids, MI.: Zondervan Academic, 2018), xliii
THE IMPLIED AUTHOR
Who wrote Isaiah? The text never claims that Isaiah son of Amoz wrote anything. (The reference in 8:16 is ambiguous on this point.) He is mentioned in the titles in 1:1, 2:1, and 13:1. He is the subject of narratives in 7:1—16, 20:1—6, and chaps. 36—39. A prophetic scribe could have recorded his ministry in these narratives. However, early readers took Isaiah son of Amoz as the writer of the entire book. But scholars and readers, especially in the last two centuries, have had diffculties with this assumption because the book deals with some historical events at least two centuries after the time of Isaiah son of Amoz. Since Duhm (1892), interpreters have thought of a writer called "Deutero-Isaiah" for chaps. 40—66, but they continue to think of Isaiah son of Amoz as the writer of "Proto-Isaiah," chaps. 1—39. Now Williamson (Book Called Isaiah) has contended that "Deutero-Isaiah" had a major role in editing "First Isaiah." The Vision's implied author, known as "Isaiah" throughout the centuries, is more nearly like the "Deutero-Isaiah" or perhaps the "Trito-Isaiah" of recent scholarship.