Why are there so many chapters in Second Nephi that seem like they are copied from Isaiah? Because, from 2 Nephi 12 to 24, Nephi is quoting the Brass Plates, which today is the Bible. He did so since “[his] soul delighteth in the words of Isaiah” and “[his] soul delighteth in plainness”. But Isaiah does not sound “plain” to us! Why? 2 Nephi 25 answers that. Verse one says: “For behold, Isaiah spake many things which were hard for many of my people to understand; for they know not concerning the manner of prophesying among the Jews.” And verse five quotes: “and mine eyes hath beheld the things of the Jews … there is none other people that understand the things which were spoken unto the Jews like unto them” Nephi understood because he was with the same group of people Isaiah prophesied too.
https://www.churchofjesuschrist.org/study/scriptures/bofm/2-ne/25?lang=eng&verse=#p