I don’t know everything about unions but I grew up in an IBEW family.
Union members work under a contract with the employer. That contract gives the workers the right to strike if a new contract satisfactory to the union has not been signed. They do not get paid while striking, and temporary workers can fill their positions if they are willing to brave the thugs that block entrances to the business. But the union workers cannot be fired and must be allowed to return to work if they want to. Labor laws allow the union employees to disrupt business by blocking entrances and intimidating customers, vendors, etc. as long as they do not actually enter the property. If a business goes non-union, other unrelated unions like teamsters will also try to sabotage that business by refusing to deliver freight, catering, or whatever to that business. They won’t cross each others picket lines. Also, most government contracts require that the contractor be union — which in the case of Verizon means that the company would lose all those contracts if it tried to go non-union.