你的问题中提到的in which是属于“介词+关系代词”型的定语从句.
关系代词在定语从句中做介词宾语时,从句常由介词 + 关系代词（只用 whom /which）引导
The school (that/which) he once studied in is very famous.
= The school in which he once studied is very famous.
Tomorrow I will bring here a magazine (that/which) you asked for.
= Tomorrow I will bring here a magazine for which you asked.
We'll go to hear the famous singer (whom/that/who) we have often talked about.
= We'll go to hear the famous singer about whom we have often talked.