I've read a lot of information about PRNGs, PRFs and KDFs. As far as I know:
PRNG: The seed for the PRNG must be a uniform string. It transforms a short real uniform string into some bits of pseudo-randomness.
PRF: The PRF gets a uniform key and some additional input and transforms it into a string of pseudo-randomness.
KDF: For example the HKDF scheme uses the hmac as PRF and does not need any uniform input string. It generates some pseudo-randomness.
So far, is that right? I would like to get some additional information about literature on that topcs, in particular further on the inputs of PRF and KDF.