Imagine if there were 10 different smartphone suppliers but all the suppliers agreed that they were only going to let you buy from 1 of them - company X. Company X would just charge you whatever the hell they felt like because you would have no other options. It would fricken suck. It would feel the exact same as a monopoly.
Now compare this to a union. Company Y's employees are all unionized and the union forbids company Y from buying labor from anyone but them. Even if it's not forbidden , new hires are typically glad to join the union too, and in some cases the law forbids otherwise. See how similar that is? It's still a monopoly - just a monopoly against a single buyer.