There was and is still today the Angelic Conflict happening. There was life here on earth before Father made man the first Adam. Exactly whether the woman he went over the hill and married we, I have no clue and the Bible does not go there to explain it. It is a "Cliff note" to conjecture, spitball off of. As the entire Bible is full of "Cliff notes, man makes doctrines out of and gets off track
As Paul stated to me in the Bible, the foundation is the Deth, burial and resurrection of Jesus as Doctrine
So, where she came from, and how, I do not know. Except this earth is very old, and man was not made on it at first from God Father, known as Father now. No info on that but that is was dark when God came upon earth and said, let there be light (referencing Jesus Christ the light) that took many years to see that truth thanks, trusting to help you, yet maybe not.
Then we have the flood that wiped out all on here on the earth then in the line of Seth to Adam and Eve. Left Noah and his wife and three Son's with their wives, and yet the Nephilm continued on
The words of God was given to a man not to understand the meaning of it and to fulfill it but to have the knowledge of sin.
ROMANS 3:20
20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.