Standard salary rates are usually negotiated by those dominant unions for the entire sector (though in the last decades, a lot of this stuff has been deregulated, so the salary rates have become less relevant today, except for state employees)
Employees are not required to join a union - however in every company above a certain size, they have the right to a work council. Unions can initiate an election for such a council even against the orders of the company's management. While the unions can enforce creation of work councils and usually offer ongoing support, the councils are generally independent and staffed by company employees.
See Ver.di vs Twitter for a current example where a work council is founded in opposition of management (text in german) : https://www.verdi.de/themen/recht-datenschutz/++co++43ec9312...
In the US a company may have employees each in different unions.
Consider a Hollywood movie studio, where the writers, actors, stage workers, and truckers may all be in different unions.
The US unions are by trade, and I think that's what l3uwin also means for NL.
That said, multi-unionism is indeed more common in Europe, if my reading of Google Scholar is correct. Eg, "Multi-unionism and the Representation of Sectional Interests in British Workplaces" at https://journals.sagepub.com/doi/pdf/10.1177/104346311454631...
> Unlike in the United States, exclusive jurisdiction is exceptional in Europe (Visser, 1992), and unions commonly negotiate with employers simultaneously with other unions on the level of the sector as well as at the company level (Akkerman, 2008). The majority of the European Union member states have multiple labor union confederations, organizing members along political, religious, or occupational (status) lines (Eurofound, 2014). Even in the United Kingdom, Austria, Ireland, and Latvia— countries with just one union confederation—unaffiliated unions and union representation by several unions within the same confederation exist. Thus, multi-union bargaining is a common if not dominant feature of Western European industrial relations. Sometimes, these unions coordinate their bargaining activities, for instance, by setting joint wage demands and synchronizing the communication to their rank and file. Joining forces increases their bargaining power.
In Europe there are multiple unions you can join for an employer. This competition keeps them working well.
In the US it's a single union per employer, and if the employer has one, you are forced to join it (and in some states you can decline to join, but still be required to pay fees). This causes lots of problems with corruption, and the union is more interested in their goals, than yours.