You could look for a job in big cities where workers are better paid.此处为何不填Which或that?

前面是cities 指代的是地方 就用where

关系代词和关系副词的选用主要取决于从句中的结构是否完整。结构完整则选关系副词或“介词+which/whom”，结构不完整(如缺少主语、宾语等)则选关系代词。
从句workers are better paid结构完整，所以应选关系副词而不用关系代词that/which，先行词是地点，所以关系副词用where。

先行词big cities在从句中做的是地点状语，不是主语或宾语，所以不能用which或that。相当于workers are better paid in big cities
但是可以加in就能用which了。You could look for a job in big cities in which workers are better paid.

city 具体地点了~

你好:
先行词是 cities 在主句 You could look for a job in big cities 中 作的是状语
先行词在从句中做状语时,应选择关系副词 而 which 和 that 都是关系代词 所以用 where 再者, where 就其实际意义 "哪里" 也是表示地点的 这个与cities 是比较合理的搭配