Complex sentence: The majority of the 40,000 plus New Zealanders trained by the RNZAF were aircrew, because most were sent to Europe, and served in squadrons where the ground crew were from the United Kingdom.
Split into simpler sentences: The majority of the 40,000 plus New Zealanders trained by the RNZAF were aircrew. Most were sent to Europe, and served in squadrons where the ground crew were from the United Kingdom.