This is the ring on which she spent 1000 dollars.为什么which前加on?为何这么用?

Question

This is the ring on which she spent 1000 dollars.
为什么which前加on?为何这么用?

Answer 1

on which 可以替换成what,on 添加到最后
spent on 是一个词组,宾语从句主语是which时on提前就是on which,同样还有in which at which