"The bus comes from that factory in which those workers are working."
Translation:A busz attól a gyártól jön, amelyikben azok a munkások dolgoznak.
That is one of those widely used incorrect usages. I am for not accepting it, but many many people speak like that.
"Ami" would be correct if the noun ("gyár") were not named/mentioned in the sentence:
"A busz attól jön, amiben azok a munkások dolgoznak." - The bus comes from what those workers are working in.