Pronouns are words used to avoid repetitions of a noun {see Subject Pronoums, A1 Level}.

Relative pronouns  are used to introduce relative clauses. They refer to people or things  by replacing the subject expressed in the main clause to avoid the  repetition of it in the relative clause {see  Relative Pronouns, A1 level}.

https://open.books4languages.com/english-a2-grammar/chapter/relative-pronouns-of-people-and-things/