本句中的in which=where,但是不能认为他们总是相等!
关系副词when、where、why等都可以根据具体语境转换成“介词+关系代词which”,介词的选择主要依据从句里动词和先行词之间的逻辑关系来决定,例如：
I will never forget the day when (=on which) I joined the army.(因为join the army on the day,所以就等于on which.
I will never forget the year when (=in which) i joined the army(同理）
My grandparents lived on the farm where( on which) I spent my childhood(因为farm前面我们要用介词on)
如果理解了,如果还有疑问,欢迎追问.