Why do we need "that" for 'the small amount' and we don't need 'that' for 'little'?
what litte remains of the enclave's food, fuel and water supplies
The idea is not just a word-level claim that "that + which" = "what". In "what little remains ..." we have an NP "what little" serving
simultaneously as the head of a larger NP and as the wh-phrase at the beginning of a relative clause. Externally, "what little remains ..."
behaves like an NP; internally, it looks just like a relative clause (though with a wh-word "what", not ordinarily allowed in relative
clauses in Standard English).
I think that's a better explanation.