We do not know the precise content of the Brass Plates. We know that they included quotations from prophets that are not in our Bible, for example, Zenos, Zenock, Neum, and Ezias, where we see that these prophets testified plainly and specifically about the coming mission and atonement of Jesus Christ and about important elements of the Plan of Salvation, including the nature of the creation and the fall as well as the nature of the Godhead.
In addition to these four prophets the Brass Plates likely contained some version of Genesis, Exodus, Leviticus, Numbers, Deuteronomy, likely some version of the history of Israel from the time of Joshua down to Zedekiah (as in Judges, 1–2 Samuel, 1–2 Kings), books of prophets such as Isaiah, and maybe Hosea, Amos, Micah, Nahum, Habakkuk, and Zephaniah, Jeremiah, some of the Psalms, as well as some lost texts, and others. Certain parts of our Old Testament, such as the books of Daniel and Ezekiel, had not been written at the time of Lehi, so they would not have been on the brass plates.
Book of Mormon Central, "What Parts of the Old Testament Were on the Plates of Brass? (1 Nephi 5:10)," KnoWhy 410 (February 22, 2018).
Millet, Robert L. "The Plates of Brass: A Witness of Christ." Ensign 18, no. 1 (1988): 26–29.