Robert Parsons notes that while most Book of Mormon readers follow Nephi's teachings in 1 Nephi 22 and recognize he is interpreting Isaiah, they often cannot determine where he finds his source material for the interpretations in his Isaiah quotations.
If we go back and check 1 Nephi 20-21 (compare Isaiah 48-49) for the basis of Nephi's teachings in chapter 22, we realize that in addition to what Isaiah says, Nephi has added some of his own understanding to make his commentary as plain as it is. We must remember that Nephi had an extensive knowledge of scripture; not only had he read Isaiah but also Zenos, Zenock, and Neum. He had also had his own visions in which he had seen the time of Christ (1 Nephi 11), his own people in the promised land (1 Nephi 12), the restoration of the gospel (1 Nephi 13), the building of Zion (1 Nephi 13), the destructions preceding the Second Coming (1 Nephi 14), and the establishment of the Millennium (1 Nephi 14). Consequently, Nephi could clearly elaborate on and explain much of what he quoted from Isaiah. [Robert E. Parsons, "The Prophecies of the Prophets," in The Book of Mormon: First Nephi, The Doctrinal Foundation, pp. 275-276]