What is a worker's union? How does it internally work? How can a union make the affiliated company do stuff to benefit the union(why can"t a company just say: f*ck off to their demands)?

well its all dependent on negotiating power of the union to cost the company profits if they strike and the trust in union that the company will not violate any contract they sign. The whole thing is dependent on the ability of the union to effectively strike and it doesnt always work and companies are willing to take a loss sometimes