That's where the best jobs are. that指代downtown。市中心是最好的工作所在之处。市中心才能找到最好的工作。

where为特殊关系副词（融合型关系副词）引导名词性从句作表语。相当于与一个名词+关系副词where引导的定语从句。 where=the place where.

系词be如果需要翻译出汉语的“是”，则表语是名词性表语，从句也是名词性从句。

He is a doctor. 他是医生。

形容词和副词作表语，be不翻译。

She is beautiful. 她很漂亮。

She is there. 她在那里。

where可以引导四种从句：

1 地点状语从句：传统语法认为此时的where是从属连词，不作成分。生成语法认为这种where是融合型关系副词（fused relative adverb），意为“在。。。的地方”。

Put the book where it should be.

Where there is smoke, there is fire.

2 定语从句：where为关系副词，指代主句的时间名词，并在从句中作状语。

This is the house where I lived for 3 years.

Last year I visited Kennia, where I saw many wild animals.

3 名词性从句：where为疑问副词（连接副词），保留疑问含义“哪里”。

The question is where we can get the money.

I don't know where he is now.

4 名词性从句：where为特殊关系副词（融合型关系副词），where=the place where. 整个从句行当于一个被定语从句修饰的名词词组。意为“。。。的地方”

This is where his brother was killed.=This is the place where his brother was killed.

Where there used to be a factory is now a beautiful square.

网友的句子就是第四种情况，where引导名词性表语从句，相当于一个带有定语从句的名词词组。