In parts of Europe, individual workers choose their unions. You don’t necessarily join a union simply because it’s the one that is exclusive to the company. Instead of fighting myriad individual battles to unionize workplaces, workers join independent unions, the unions collaborate to bargain in a sector, and set things like wage floors and working conditions across the industry.

The US specifically did not want this to happen, which is why it made unionization only valid on the basis of the adversarial, shop-vote model, but grandfathering in some unions that already functioned along similar, sectoral lines, such as the Hollywood unions.

yes unions are mostly big and exists across companies. However you are also free to join any union you like or none at all. For jobs a restaurants, factories etc. salary is mostly the product of Employer - Union negotiation. As a software dev, you negotiate yourself (unless you work a gov job).