"on which" 和 "in which" 都是介词短语，但它们用法上有些不同。
"on which" 表示某物或某人所处的位置或状态。例如：
The book on which I am reading is very interesting.（我正在读的书很有趣。）
The table on which the vase is standing is made of oak.（花瓶所放的桌子是橡木制的。）
"in which" 则表示某物或某人所处的位置或环境。例如：
The house in which he lives is very old.（他住的房子很老。）
The city in which I was born is located in the northeast.（我出生的城市位于东北部。）
总的来说，"on which" 更强调所处的具体位置，"in which" 则更强调所处的环境或位置。