Is this factory( that)we visited yesterday?（这个工厂是我们昨天参观过的那个工厂吗?）
Is this( the factory which/that)we visited yesterday?（这是我们昨天参观过的那个工厂吗?）
两句话主语不同,分别为this factory和this
你可以把它还原为陈述句就明白了啊：
第一句变为：
This factory is where we visited yesterday.
第二句则是：
This is the factory which/that we visited yesterday.