I don't know the Society's official stance on this, but in the 1990's and early 2000's, I heard some JW elders give these answers before:
1: Jehovah created the spiritual Heavens out of the same "holy spirit energy" material (or out of nothing) that He used to create everything else. Before this, there was just God, nothing else.
2: Somehow Heaven (the spirit realm) is eternal just like God is eternal.
3: God lives in outer space but deep, deep, deeply beyond anywhere that we can ever discover.