Genesis 7:2 Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female.
Great instructions about the clean and unclean animals will be found in Leviticus 11. That will be years after this chapter but Noah already knew about clean and unclean animals.
So much of what we find in the Bible is not showing all the details. How did Abel know what offering God expected and demanded? Why would God get angry with Cain if he did the best he knew how and could?
The answer is that Adam and Eve were taught lots of things that aren’t written in the Scripture but revealed in little tidbits all through. For example, as Noah prepares to build an ark to save the world he is told to take on a certain number of clean animals compared to other.
What were clean animals? What were unclean animals? I think it is fair to assume that Adam and Even knew and taught their children. Noah just knew!
So the question is, are you teaching your children? Do you realize that this might help explain some of the questions you have about sacrifices and other things that seem to happen before it was written?