Re: Can someone please explain...
It gets messy and I'm not entirely sure of the precise details, but approximately an employer will agree with a union to only give work to members of that union. Want a job? Join the union.
This is an interesting read into the impacts of unionisation and includes examples of unions demanding that non-union members are not employed:
https://www.studiobinder.com/blog/working-with-unions-flipped/
Suffice to say, I'll never make a film in California. Why would I want to deal with that nonsense?