It depends heavily on the country but a worker at a company can pick a union to represent them. You might see a billboards saying join Union a or Union B. If you Union a is corrupt or screwing over one group of workers in favor of another group of worker, the laborers will just leave it and go to a different one.

This solves a lot of problems with us unions, where they have a state sanctioned Monopoly on the workers.

The classic example is unions are going for terms that screw over junior members but there are other perverse examples. At my friend's company the Union demanded that it be forced to return to office because the larger number of field technicians we're a voting majority of the union and angry they couldn't work from home.