it is quite obvious that Genesis was written by different hands due to style and content, to say nothing of the covenants around which it was written. For example chapter 14 is totally different from the rest of Genesis, and calls Abram 'the Hebrew', suggesting it was compiled by a foreigner ('the Hebrew' was a foreigners way of seeing the people of God). It dates itself by foreign rulers.
We have to ask, 'why was Genesis written around covenants'? And the answer is because they were the main reason for very early writings. Why was Isaac written about so little? Because he had few covenants.
Furthermore the phrase 'these are the family histories of --' were typical of a colophon added to a tablet to show what it was about or who wrote it.
In those days the fact that Moses took earlier tablets and incorporated them into his text would have been quite normal.