This is not a relative pronoun. You correctly identified that the verb order is not the one that would be found in a relative clause.
The part before the comma is in one case the subject of the sentence ("Wer ist gut?" - Die Milch von Müller), in the other case the object of the verb ("Was wollte er?" - Den Apfel essen). In your examples, it is once a noun phrase (Die Milch von Müller) and once a verbal phrase (Den Apfel essen), which you correctly specified as "Infinitivkonstruktion".
The pronouns are subject or object pronouns respectively. The pronoun can be removed, and the sentence still makes sense:
Den Apfel essen wollte er.
Die Milch von Müller ist gut.
You could remove the reference from the sentence, and keep the pronoun instead:
Das wollte er.
Die ist gut.
This demonstrates the relation between the pronoun and its reference. You could consider them demonstrative pronouns:
Demonstratives are often used in spatial deixis (...), but also in intra-discourse reference (including abstract concepts) or anaphora
It might also help to translate the sentences and mark the pronoun.
Eating the apple, he wanted that.
The milk of Müller, it is good.
Notice that the first translation is not idiomatic in English. The translation just intends to make the grammatical structure in German more lucid.