They work in large groups, grouped by discipline (topic). It's a voluntary effort. Anyone can join. I've been active on some of the development committees. Whoever "shows up" from CA colleges and universities joins the discussion and provides input.
While the idea of reduced-cost or free textbooks for students sounds noble and appealing, these "collective" writing efforts can have a political bias. That is the obvious danger. There is usually one "free" text per topic, such as Macroeconomics, so it is a monopoly product. No alternative free texts offering different perspectives are provided to students, right now.
Just be aware of this - it is especially concerning in social science subjects, like mine (Economics), as well as for world history and US history, political science, etc. Even physical and earth science "free" texts can harbor political bias (e.g., man-made global warming).
IOW, 'You get what you pay for' still holds... Yes, textbooks are enormously expensive. $100-$200 each and up. But -- there ARE ways an instructor can "diversify" the student's reading list so they become critical thinkers, at a low cost -- for ex., the Mises Institute has free downloads of classic Austrian works students can be assigned, etc.
I'd encourage conservative professors, both adjunct and full-time, at colleges & universities to get involved in these online text collaborations and provide strong input. Or, start your OWN group - and develop an alternative open-source text offering conservative viewpoints & perspectives.
What really caught my eye at the textbook site were the math sciences. An opportunity for smart kids to access college level math, for the highschool dropout to bootstrap themselves and for the elderly to challenge their minds again. Free. From their couches. Equally amazing.
U.S. History covers key forces that form the American experience,with particular attention to issues of race, class, and gender
I learned long ago that "unbiased" is a myth. It goes against human nature.
One of the greatest propagandists in history was Benjamin Franklin.He created completely "fake news" about colonials starving and their children dying and forged the bylines from reputable American newspapers to convince the English not to seek reparations for the war.