They’re part of ver.di, although that in itself skips a whole host of details that I personally don’t all know either. But yeah over here plenty unions are local in a company, too. But they also got strong legal protections (and some obligations), so apart from the shakey time when you’re forming a union, they’re on stable ground.

Some others - like ver.di - are large cross-business union aggregations that centralize their decisionmaking process and then can strike on multiple divergent businesses at the same time to force business owners to come to the table. From what I know from people who are in such companies, upsides and downsides. They got a lot of pull, but in return their demands are often rather generic which is good if you just care about pay, but bad if you want things more specific to your work.

But again, totally not an expert about unions.