If you can get access to it, I highly recommend the Paris Review interview with Billy Wilder for a good insight on how the studio system used to function. A snippet:
Wilder: When I was a writer at Paramount, the studio had a swarm of writers under contract--104! They worked in the Writers Building, the Writers Annex and the Writers Annex Annex. All of us were writing! We were not getting big salaries but we were writing. It was fun. We made a little money. Some, like Ben Hecht, made a lot of money. All the writers were required to hand in eleven pages every Thursday. Why on Thursday? Who knows? Why eleven pages? Who knows? Over a thousand pages a week were being written.