The Subject is the “geborene Sohn” … so is “der am 23. Oktober 1973 in Hamburg” a relative Clause?

Question

Dabei entstammt der am 23. Oktober 1973 in Hamburg geborene Sohn eines Arbeiters und einer Schullehrerin einem fast schon bürgerlichen Milieu, geprägt von der ersten Generation Einwanderer.

Answer 1

No, it's all one lengthy subject.

Dabei entstammt der am 23. Oktober 1973 in Hamburg geborene Sohn eines Arbeiters und einer Schullehrerin einem fast schon bürgerlichen Milieu.

Let me rearrange it:

Der [...] Sohn [...] entstammt dabei einem fast schon bürgerlichen Milieu.

German speakers aren't afraid of such lengthy subjects and objects.

Answer 2

Analysis on top level:

1. dabei
   Konsekutives Konjunktionaladverb (consecutive conjunctive adverb)
   This word links the sentence to what has been said in the sentence before. (This previous sentence is not available here.)
2. entstammt
   Verb (verb) (the singular, 3^rd person form of »entstammen« in Gegenwart = Präsens (present tense))
   What ist the action? What is going on in the sentence?
   In every German sentence that is a statement (i.e. neither a question nor a command) the verb has to stand on position 2.
3. der am 23. Oktober 1973 in Hamburg geborene Sohn eines Arbeiters und einer Schullehrerin
   Subjekt (subject)
   Who is performing the action?
   Here we have 14 words, which together form one noun phrase (in German: Nominalphrase), and this one phrase is the subject. The head word of every noun phrase is a noun (that's why its called noun phrase), and the grammatical properties of this head noun define the properties of the phrase.
   In this example the head word is »Sohn« (son). It is singular and masculine and stands in nominative case, and so the complete noun phrase also is thought to be masculine, singular and in nominative case.
   Since there is nothing else in this sentence in nominative case, and because every sentence must have a subject (with rare exceptions), end because the only case in which the subject can appear is nominative case, this 14 word noun phrase must be the subject.
4. einem fast schon bürgerlichen Milieu
   Dativobjekt (object in dative case)
   The verb (on position 2) needs some complements. Each verb needs its own set of complements. One complement, that every verb needs, is the subject. But all other complements depend on the verb.
   The verb entstammen (to stem from) needs a subject, that tells us who is stemming from somewhere. And it needs a second complement, that has to stand in dative case, that can tell us the source, i.e. from where the subject stems. This source is this object here.
5. geprägt von der ersten Generation Einwanderer
   Nebensatz (dependent clause)
   This is Verberstnebensatz (dependent clause with verb at position 1) that is derived from a relative clause (»das/welches von der ersten Generation Einwanderer geprägt ist«)

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The parts of speech on the positions 3 to 5 can all be separated deeper into their parts, but according to your question, I only do this for the subject, i.e. the part of speech o position 3:

• der
  The article of the head noun (which is »Sohn«)
• am 23. Oktober 1973
  A Präpositionalobjekt (prepositional object), here in the role of a temporal attribute of the adjective (which is »geborene«)
• in Hamburg
  Another prepositional object, but this one is a local attribute of the same adjective
• geborene
  An adjective (the weak declined singular masculine positive form of »geboren«), which here is used as an attribute of the head noun
• Sohn
  The head noun of the noun phrase. Masculine, singular and in nominative case
• eines Arbeiters und einer Schullehrerin
  A list with two items. All items in the list are of the same kind, they are genitive attributes (genitive objects, used as attributes of the head noun)

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Both prepositional objects consists of a preposition and a dative object:

Note, that »am« is just a short form of »an dem« (like don't = do not)

• an dem 23. Oktober 1973
  The word an is a local preposition, the rest (dem 23. Oktober 1973) is an object in dative case
• in Hamburg
  In is a local preposition, Hamburg is also a dative object.