定语从句中，到底是引导词that前面不可以加任何介词还是引导词that前面不可以加前置的介词？

比如I have read all the books, three of which/that were assigned by the professor last week.（这里引导词可以用which或者that，grammarly检测都无误）

还有Julie was good at German, French and Russian, all of that she spoke fluently. 

这两句都没错，而像We depend on the land from which we get our food. 这种，介词from前置了的，引导词就只能用which不能用that。

我的理解正确吗？