Ah, that makes sense, then I might have misunderstood.
So does that mean, when the headlines say "Amazon workers (from a particular warehouse) vote to unionize", it's really about bringing that warehouse under the umbrella of an existing union?
Oftentimes, yes. They could form their own union but it's common to join an existing union.
Having universal or near universal union membership for a certain profession is limited to certain jobs, things like warehouse workers or retail employees aren't anywhere near that.
So does that mean, when the headlines say "Amazon workers (from a particular warehouse) vote to unionize", it's really about bringing that warehouse under the umbrella of an existing union?