Under these arrangements, unions act as the exclusive bargaining representative for all employees in a unit, whether they join a union or not, and all employees, including non-union members, are required to pay a share of the unions’ collective bargaining costs. (Under Abood, non-union members are not required to contribute money for political activities or other union activities not connected to collective bargaining.)
https://www.theusconstitution.org/litigation/friedrichs-v-california-teachers-association-u-s-sup-ct/