The modern concept of Union is able to accomplish this by the force of sword and gun. Laws have been passed that allow Unions to access the police force of the state to intervene on the Union’s behalf. With this access, the Unions are able to negotiate benefits for themselves that would not otherwise be possible in a relationship between two parties. In short, the Union uses legal coercion to obtain their benefits.
And this increases unemployment and raises the cost of good above what they otherwise would be.