Depends on the country.

In a lot of countries, it's not much different than choosing a pension fund option. You join a job, you choose which union best represents you based on your industry/title/etc (if you are not already a member). If you are not happy with your current agreement, you choose a different union that promises better outcomes.

It's so much less extreme than US unionization. There's not much reason for hostility between business and unions - unions don't get these all or nothing powers like they do in the US.