DO SO 可以指代句子的一整个动词，包括它的宾主，修饰语：

Quinn did not eat dinner quickly, but her brother DID SO.


在这里，do so 起的作用就相当于一个代词，而有时我们也省去so:

Quinn DID NOT eat dinner quickly, but her brother DID.


DO IT 则就要明确指前面的名词：

Quinn failed to do the homework, but her brother did IT.

It 指向了the homework，而这句话可以省略do