After welding school?

Hello, I am going to school for (look at photo). I keep hearing to join a union after, but when I look up the local unions it says about a 5 year apprenticeship and that it can take up to 2 years to be accepted. After I finish schooling, do I just apply to all unions? And will having a certificate give me a better/quicker chance of joining a union? Thank you in advance.