The way it works in the movie industry is actors or writers can sign a contract with minimum union terms. Or, if they're a big name, their agent negotiates a contract on their behalf.
From time to time the union membership will want improvements or changes to the minimum terms. If they don't get these terms then the union - stars and everyone else - goes on strike.
These strikes are well publicized. I'm surprised you haven't heard of them.
You don't have to care. None of the parties involved care if you care or not. On the other hand, if you had an open mind about this topic, you'd see that strikes work based on this evidence.
Does collective action mean everyone gets paid the same? If not, how does that work exactly?