It's false to say that it "uses the definite article". It uses a definite relative pronoun that shares several forms with the definite article. But "denen" and "dessen" never occur as forms of the definite article.
Similarly, the indefinite relative pronoun shares most forms with "wer". To emphasise it's indefiniteness, you can follow it with "(auch) immer". E.g. "Wen auch immer Sie sahen, er war nicht ich."
"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons