1. Birth in the human realm starts with the birth of the gandhabba.
– That “human existence” can last for thousands of years, i.e., the lifetime of the gandhabba can be many thousands of years.
2. To be born with a human body, that gandhabba needs to be pulled into a womb. Then after 9 months, a baby is born.
3. That baby grows to be an adult and dies within about 100 years or so. But the gandhabba does not die. It comes out of the dead body and could stay in that state for many years until pulled into another womb.
– That leads to another birth with a physical human body, which in turn dies after around 100 years.
4. That process continues until the gandhabba dies at the end of its lifetime.
5. At that point the “lifestream” grasps a new existence (Deva, Brahma, animal, hell-being, etc.)