terminology - How are relative pronouns pronouns?

Pronouns usually replace nouns or noun phrases. There are a few fuzzy examples where it's not as clear cut as that (e.g. "my" which refers to me but acts like a determiner, and "mine" which refers to both me and something else).

How are relative pronouns like that and which pronouns? What noun or noun phrase do they replace? If they're not strictly speaking pronouns, what would be a better description for them?

Examples from Wiktionary:

• The CPR course that she took really came in handy.


• The house that he lived in was old and dilapidated.


• We've met some problems which are very difficult to handle.

I'm not talking about other clearly pronominal uses such as:

• Which is which?


• That is that!