in which:在......里面；on which:在......的上面，或具体时间的某一天

on which 和 in which 都可以用来引导定语从句，修饰先行词，区别在于介词on/in 这取决于先行词的搭配

比如:

I can't forget the day on which we had a party.这个句子，先行词是day，原本的搭配是on the day.

I like the room in which there are many toys.这句话的先行词是room，词组搭配是in the room。