Thank you for the question on the authorship of Moses, “How could Moses have obtained the material he incorporated in the last part of Genesis?”
The Biblical text does not state how Moses received the content of Genesis. Here are some possibilities:
A. Through oral tradition of the forefathers.
In most nomadic culture, the family history was passed on through verbalization by the chief of the tribe/clan or by some religious person. This view would adopt that Moses wrote down the most prevalent view at that time. It was an arbitrary one or he modified the the oral traditions of the forefathers.
B. Through the writings of Joseph.
Joseph became the prime minister of Egypt. He was responsible for the administration of the Egyptian country. As prime minister, it is within the realm of reasonableness that Joseph would recorded what his father, Jacob told him. This record was preserved even as his body was preserved to be taken back into the Promised Land. Joseph’s record could have been cited in the writing of Genesis by Moses. This is speculation on my part.
C. Through divine inspiration by God.
Just as God gave the various details of the Law and the detailed blueprint for the building of the Tabernacle to Moses, so it is also possible that God dictated the history of Jewish people. Moses had forty years to write the history and the Laws for the Jewish people.
D. Through a copyist/s who forge the fake history of Israel.
A copyist or a religious leader forge the history of Israel during the Exile of the Jews in Babylon. It is attributed to Moses but Moses didn’t write any of it.
Those are the views of how Moses came to write the Book of Genesis. The reader has to assess and determine which of the views is more likely than the others.
-Kingston