这里的 so 与 so do I, so did he 之类的用法是一回事。so 的意思是“ 也，同样“，用于表示刚说的情况对某人或某物也是如此。这样用的 so 在意义上相当于too, also, in the same way，在句中作状语。

原句的翻译似乎不准确：

So are remote-controlled cars which are kids' favorite

= Remote-controlled cars which are kids' favorite are also made / created in China.

其意为：青少年们最喜欢的遥控车也是中国制造的。

联想并比较：

He likes this website, and so do I.

=He likes this website, and I like it too.