英语语法the existence of a city is often longer than that of the

英语语法
the existence of a city is often longer than that of the empire of which it is the capital
城市的存在通常比以他为首都的帝国长
这里的两个语法点我不太明白,一个是that of,that指代的是exsistence(参考书上这样说的),还有which it is the capital,it我觉得是指代的city,但是这又是什么语法呢?
比如说还有一个句子,a city's peculiar qualities and characteristics are sometimes those of the people who live in it.
those of是一样的，这些代词在这里是什么语法？