"in which" is not a grammatical unit in itself. You don't need to learn how to use it - it's simply the preposition "in" followed by the word "which". The relative pronoun "which" can follow almost any preposition, such as by, to, with, from , of , on , at and many more. I was born in Cairo = Cairo is the city in which I was born I was born in May = May is the month in which I was born

This gives a pretty good breakdown: http://www.grammar-monster.com/lessons/which_that_who.htm And no, "which" and "in which" (or "by which", "through which", etc.) are quite different from each other.