> So who owns the the actual playwright?

Do you mean copyright?

>Does an OG producer sell these rights to another?

Generally the corporation that pays for a movie or tv show is considered to be its author or owner.

1st example:

Jim Gavin wrote the pilot for a show he called Lodge 49.

Paul Giamatti and Dan Carey read it and loved it.

They got the script to Susie Fitzgerald at AMC.

Susie Fitzgerald convinced her bosses to film it as a 10 episode series.

Even though Jim Gavin created the show and oversaw its writing, AMC became the author/owner of the series when it agreed to underwrite its cost. However, Gavin has used elements of the show to brand and promote his publishing house. He's negotiating with AMC for permission to publish a series of novels based on elements of the show.

2nd example:

Louis CK wrote a 10 episode series.

He hired actors/etc and filmed it.

He released it to his people who were subscribed to his website.

After a period of time, he sold it to Hulu.

Hulu became the author/owner of the series.

>Or does the studio basically have all ownership?

In both cases these creators signed contracts with the studios who produced/bought their shows. These contracts refer to "rights." Creators' rights to the work they create follows the terms of a contract.

3rd example:

Ben Edlund created a comic book character called the Tick in the mid 1980s.

He signed a contract with New England Comics - who've published several hundred comic book titles based on Edlund's ideas over the last ~35 years.

In the early 1990s, Edlund signed a contract with Fox to produce an animated adaptation of the Tick. It ran for 3 seasons. Fox promoted the show with fast food items, t-shirts, toys, a video game, a board game, and other sundries. This was all negotiated in Edlund's contract with Fox.

In the early 2000s, Edlund signed a contract with Fox to produce a live action adaptation of the Tick. It ran for 10 episodes. Edlund's contact for the animated show prevented him from using several of its most prominent characters so he invented new ones.

In the mid 2010s, Edlund signed a contract with Amazon to produce another live action adaptation of the Tick. It ran for 20 episodes and featured a new slate of supporting characters.

In each case, Edlund retained control over certain elements while the corporation underwriting production controlled others.

The catch is that all of this is negotiated by executives working for the corporation and agents/lawyers working for the creative team. The bigger or more expensive the project, the more complicated this negotiation becomes.

4th, final example:

Steven Conrad created a tv show called Patriot for Amazon. When Amazon cancelled it, he got one of its stars to record an audiobook version of the book that actor's character had written as part of the show. Conrad did this without any permission from Amazon.

I don't know if that helps clarify. I can say that one indelible part of Hollywood culture is that people rarely share details of these negotiations. I guess it is considered gauche.