How to use what, that, who, where, etc. in Chinese?
What you are talking about are called relative pronouns. Chinese, you may have noticed, does not play by English rules. Chinese does things differently, and to my mind often more elegantly. One way is this use of 是 ...... 的 or some other 动词 .... 的 combination. Relative clauses are just bigger adjectives really, and that is often exactly how Chinese uses the information content of the relative clause, as part of a big adjectival phrase, in front of a noun.

[relative clause]
(information content of relative clause)
{noun + relative pronoun, or just relative pronoun}  


That is {the dog [that} （bit me）]!
这是那只(咬我)的狗。
What is that chair /used/ for? (For what purpose is that chair used?)
那把椅子是干什么用的吗？


2a. Do you know {WHAT} that chair is /used/ for?
2b. Do you know {the thing [that} that chair is (used) for]?
你知道那把椅子是干什么用的吗？

Using 所 you get close to an English style relative pronoun，but here it is still wrapped inside a 是 ... 的 clause. In my head, I always think 'thatwhich' for 所。


Integrity is {WHAT} he needs.
3a. Integrity is {the thing [which} he needs]. (In Chinese I think they will say 'the thing which he lacks)
正直是他[所(缺少)]的。
I know {a few people [WHO} （drive blue cars）].
4a. I know {a few people, [those people} drive blue cars].
我认识几个(开蓝色汽车)的人。  


Don't like writing in the comments box.

Simple answer is: No. It depends on what you want to say and how you want to say it. 回家的人 is not a relative clause, just adjective + noun. There is a thing called a Zero Relative Clause. Basically, the Relative Pronoun is omitted, but could be inserted. "This is the man (who) I met yesterday.

那时正是冬天, 黑漆漆的街上, 满满都是回家的人。
It was winter then, the dark streets (were) full of people (who were)  headed home. (The words in brackets make the sentence too wordy.)

我到家的时候, 我们一起去超市吧。
¨WHEN I get home, we will go to the store¨. 

There are various ways to express 'when'. An easy way is 的时候， 的时

Chinese can put the information content of an English relative clause in front of the noun. That means, it is not a relative clause but an adjective. We can't write 'The headed home man' in English, but in Chinese it's easy. I believe this originates in the Germanic penchant for 'Verb second'.