For me personally it means Systems of Domination or institutions of subordination.
One example might be that young boys are raised up with the idea that is their birthright to subordinate women when they become "the man of the house".
That dynamic does not exist in a vacuum. It is inextricably intertwined with capitalism, white supremacy, and more systems of domination, each which we should name and confront
I laud the curiousity. Unions are diverse and within even one union exists dozens or hundreds of 1:1 relationships as well as collective overlapping interests. My summary answer thus is: No, on their own, unions are not an institution of subordination, particularly given the alternative (subordination to the boss/baron/owner), but a complete picture requires an Anarcho-Relationship lense (see the honeycomb video)