[person] + who / that + [verb] …
[thing] + which / that + [verb] …
[person or thing] + (who / which / that) + [subject] + [verb] …
A relative clause comes straight after a noun. It tells you which person or thing the sentence is about. Without it, the listener doesn't know who or what you mean.
Use who for people and which for things. That works for both, and in everyday English it is the most common choice.
When who, which or that is the object, with another subject after it, you can leave it out.
When it is the subject, with a verb straight after it, it must stay.
Where is for places: the café where we met. Whose is for "his, her, their": the girl whose bag was stolen.
The relative word replaces him, her or it, so don't add another pronoun.