The Bible begins with the only beginning that man`s mind can handle - Creator. And our Creator created everything that is.
If a person is unwilling to believe this verse, they will be unable to believe anything that follows.
Our Lord is Creator of all things and the upholder of all things by the Word of His Power. Heb 1:3
Genesis is revelation knowledge of the origin of all things. All other explanations are foolishness based on man`s rational mind and its limitations.
Though life spans greatly reduced after TheFlood, we also have Abraham being the patriarch of what would be later called the ChildrenOfIsrael - Israel being his grandson. The Children of Israel dwelt in Goshen for 400 years when Moses was born.
Moses penned the creation account which had been passed down from generation to generation by a people to whom heritage was important. I am confident that the details of the creation were filled in when Moses spoke with our Creator.