You missed the part that in some countries, even if they don't join, depending on the union structure, they would still get the benefits from the industry agreements made between the union, government and industrial partners.
This is a key component of the Nordic model. The legislatures in the Nordics choose to not legislate a minimum salary and make a number of labor laws conditional to collective agreements.
The theory is that the threat of legislation is sufficient to prevent imbalance between the parties. The state takes on the role as antagonist, making the unions and employers (somewhat uneasy) partners in trying to self-regulate. If they fail, parliament legislates, which is generally speaking perceived as an undesired outcome by everyone (crucially including most political parties).
From my Swedish point of view the model seems to work for private local employers, but struggles with the globalized tech workforce and for some public sector employees. It also seems to complicate work visa compliance by hiding away some of the terms.
Yes, in many countries in Europe, the industry wide unions negotiate with representatives of the industry employers, and government can declare the deal binding for all companies of a certain size in the industry.
Also if workers go on strike they don't get paid by their job, the union can pay them something instead from fees they received in the past.
