Outside the US there tends to be free choice of union. E.g. in Nordic countries Germany etc you normally have multiple unions to choose from and you don’t have to be member if you don’t have to.

It is the US and Anglo-Saxon countries were unions tend to be a binary thing. Germanic countries tend to follow a different union system: sector wise unions rather than company specific unions. Different types of employees will typically join a different union depending on the type of work and interests they have.

Unions are optional. There are at least 3 unions at my place, and most employees aren't members.

What's the issue?

In the united states most unions are not optional

I would go one farther. I won't join a union, and I won't work in a union shop.

If my company goes union, I'll go work somewhere else.