One of the major differences between US unions and the rest of the world is a ~50 year old legal hangover related to racism. US unions used to be allowed to moderate their support for members (as in “Joe Bag-O-Donuts is a detriment to the company and should never have been hired”). Unfortunately, many unions used that discretionary capacity to refuse to support their African-American members. It went all the way to the USSC, but the upshot is that US unions are required to have a more antagonistic relationship with management because they can’t endorse throwing out dues paying members.
Not necessarily, some are quite centralized. For example, LO in Sweden. The US has plenty of examples. Look at Harlan County for an extreme example.