I don't remember all the details but as a long time listener to Scriptnotes (if the industry of screenwriting for Hollywood interests you highly recommended) it

I don't remember all the details but as a long time listener to Scriptnotes (if the industry of screenwriting for Hollywood interests you highly recommended) it gets pretty complicated. First people get paid for the initial draft. Then they can get paid for rewrites (many contracts come with provisions to give the writer some number of drafts back and forth with the studio/director). Sometimes these can be negotiated into yet more drafts if things are close and the studio is happy with the writer. More commonly, at some point a different writer is brought in to update it, sometimes heavy rewrites, sometimes just tweaks. Once it gets filmed, the writer's guild has a process of arbitration to determine who gets credit, based on roughly who is believed to have contributed the most original material. The person or persons who are considered the primary creators get residuals based on % of money made (actual money made not the Hollywood accounting bullshit).