yoyo is right. You are not forced to take a union job. But if you have a union job, ior a non-union that that becomes one you are forced to join the union.
Once more for the record. I took a mangement job. The place voted on and became union four days later. More than a year later they settled on a contract that had less than I had gotten for them in my first four days.
And they had to pay union dues.
When I lived in Nebraska, a right to work state, the Railroad used a loose translation of the law and said that since the railroad falls under the Interstate Trade Commission it was federal and had to be unionized. I tried to opt out, to no avail. After a vehicle accident I was laid up for 3 months and upon return was told that since I had a military pension I needed no support for the time I was out of work, then they slapped me with 3 months of back dues for the time they protected me while I was out. After about 8 months of arguing with them, I went on my way with the attitude that Labor Union Leaders are not about labor forces beyond the extent of how many members they have to collect dues from so that they can fly around in private jets and live in big homes.