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.
On the otherhand, having a plethora of unions at a workplace weakens labor's bargaining power: A US union, by the nature of having a closed shop, has the power to destroy the business with a prolonged strike. Nobody wants that (capital's out their investment, labor's out their jobs), but it serves as the nuclear option from which the union's bargaining power is derived.
> The classic example is unions are going for terms that screw over junior members but there are other perverse examples.'
The story I always think of is the Port of Portland shutting down was because the ILWU organized a $20 million dollar slowdown to punish the operator. The operator ended up pulling out of the city never to return, costing all the ILWU workers their jobs, as well as the city of Portland a few million a year in taxes.
The reason for the $20 million dollar slow down? The ILWU wanted to take two jobs away on the site from the IBEW!