the easiest difference to notice is that in the US creating a union inside a company needs to be voted on, whereas eg in germany a council elected by employees

the easiest difference to notice is that in the US creating a union inside a company needs to be voted on, whereas eg in germany a council elected by employees is mandated for any company with more than 50 employees. unions are not created in the company but exist outside and any employee is free to join them at any time. if a company has enough union members they can take action there. which means growth of unions is just a matter of individual choice of each employee. and there is nothing the company can to to stop it.