That's a long question, I'd point you to the book The River of God by Gregory Riley. His 4th chapter traces the history of that question. IIRC, when Israel was in exile they encountered more dualistic ways of thinking and understanding the world. And in doing so reinterpreted this history in that light.

So you can take stories like Job, who in that context Satan is a person in God's court. But, in that context he functions more as a character in a story that an actual person we can glean knowledge about. Similarly, in other earlier OT stories, the word translated Satan, in Hebrew means adversary. Number's 22 is a great story of Balaam's donkey in v. 22 the text says there is an adversary in his way. That context it's clear Satan isn't in his way, just an adversarial character. And it was an Angel of the Lord in the way, so it's not like it was an evil character.

But, as time went on and Israel encountered, and adapted more dualistic ways of thinking they began to add an article in front of adversary. You can see this in the LXX. I used to have a good example of a story in Kings and in Chronicles that illustrated this but I can't remember and all my notes are in my office.

That's the tl:dr version, but Riley gives a better outline.

One of my fellow moderators, at /r/AcademicBiblical has written an extensive article that sheds much light on the subject at hand.

It is entitled:

Princes of Darkness: The Devil’s Many Faces in Scripture and Tradition

Isn't Satan seen as a separate evil being in Genesis with the Adam and Eve story?