"In which" is used to indicate a place, time or topic, and it is similar to saying "where" or "when". "The room in which they put the table was dark." "The year in which he was born was 1985."
"Which" by itself is used to give more information about a topic. "The woman was a doctor, which was not surprising since her parents were also doctors." "He made a grocery list which included bread and vegetables."
"Which" is a relative pronoun that is used to give additional information about a word that is in the same sentence. "In Which" is a specific type of relative pronoun that is used to give information about a place or a period. "In which" is most frequently used in more formal communication. In my experience it's rarely used in casual conversations. You might consider checking out www.sifferencebetween.com