I liked what he said.
I liked everything/all (that) he said.
I liked some of what he said.
I liked some of the things (that) he said.
'What' as a relative has the idea of 'the thing(s) that. 'All' as a pronoun has the idea of 'everything. Perhaps that explains why 'all, everything, nothing, something, thing, things' cannot be followed by relative 'what'.