this is the factory that⼀which we visited last we

this is the factory that/which we visited last week.
此句中factory 在定语从句中作的是宾语， we visited the factory，用that/whcih 也可以省略不用

this is the hotel where they stayed last week
此句中的hotel在定语从句中作的是状语，they stayed in the hotel,作地点状语用where.
希望帮到你。

可以先把这两句话不用定语从句来表述：
we visited this factory last week

we stayed in this hotel last week.
可以发现第二句比第一句多了个in,因此第二句在用定语从句完整表达时应该是this is the hotel we stayed in last week.
in which ＝where

因为是问他们住的地方,问的是方位,位置
至于which是用于讲物体,the place which attracted me most was这里的place不是表示方位,而是那个地方,跟前面那个问位置的是不一样的.