He went inside to find his keys, at which point he discovered the note.
In this sentence, does 'which' (a relative determiner) substitute for the entire clause 'He went inside to find his keys'? I am aware of sentential relative clauses, so I'm assuming that it acts in the same way.
The relative clause is a supplementary (non-defining) one in which the relativised element is the whole of the object of "at", i.e. "which point"; this is the phrase whose interpretation is given by the antecedent, in this case the whole preceding clause “He went inside to find his keys”.
In simpler terms, 'which point' refers to the entire preceding clause, which is its antecedent.