This is the 60th most frequent Croatian word.
kojoj
"Kojoj" means "to which" or "to whom" (dative case, feminine singular).
In this sentence, 'kojoj' ('u kojoj') is used as a relative pronoun referring to a feminine noun in the locative case.
Pronašao sam knjigu u kojoj sam pročitao zanimljivu priču.
I found a book in which I read an interesting story.
Here 'kojim' is the instrumental case of the word 'koji', showing its application in asking about a path or method.
Kojim putem trebamo ići?
Which route should we take?
In this usage, 'kojoj' is in the dative case, referring to the indirect object (the person) in the sentence.