So if the company doesn't sign a contract that they will hire union labor they are free to hire whoever they want? I wonder how this will work with IT since there seems to be an abundance of labor and still decent salaries being offered.

the union is free to strike if they don't sign. It's about what's less expensive for the company and who has the more leverage. If the union is well-organized, they have disproportionate power over the management and the management concedes everything.