I live in Belgium and everybody (or at least every adult) can join a union if they want to. You have a selection of unions where you can go to, regardless where or if you work. There are some that are more specialized. e.g. for train staff, for white color, for management (yes, they can join a union as well), military and there will be several that will be specialized.
I just am with one that is close to where I live, because that is what was important to me.
So I just join the union and that is about it. None of the companies I worked for cared, except one, because they paid the contribution. When people are hired, nobody will ask if you are union or not, because nobody cares. I can imagine you are not even allowed to ask, but it is not important.
Also if the company has more than 50 employees, you have to have social elections that will vote for union representatives that will have monthly meetings with management.
So we have the choice and in reality every company with more than 50 people is unionized. All of them. By law.
The advantage is that it keeps the companies in check a bit. I get my 35 days vacation. I get several weeks payment minimal when they fire me (unless I did something illegal) and if there are major cuts in staffing, they will negotiate a much better deal than I ever could. e.g. 7 months pay after working somewhere for 2 years for me. Minimum was 3.5 months for somebody who was there for 1 month and others who worked there longer it was a lot more.
So yeah, I think it is strange that anybody could forbid me to join a union or even decide that I MUST join a union, or I can not get the job. (See, works both ways)
From what I see, the US has guilds not unions. A guild is something that protects the profession. A union protects its members. At least that is the difference to me.