1. “Bhava” means the duration of an existence. A Deva and a Brahma are born once during a “Deva bhava” and a “Brahma bhava,” respectively. However, within a “human bhava,” a human can be born many times with a “physical human body,” and the same applies to an animal.
- A human gandhabba is born at the beginning of a human bhava and lives until the end of that bhava. That human gandhabba is born with a physical human body many times during that bhava.
- In contrast, a Deva is born only once during a “Deva bhava.” The same applies to a Brahma.
2. The hells (nirayas) may not be at the center of the Earth but well below the Earth’s surface.
3. It depends on what you mean by a “world.” There is a human world, an animal world, etc. There are six Deva realms, and their aggregate could be called “Deva world.” But all 31 realms combined could also be called the “world.”
4. and 5. These questions imply that you need a better understanding of what bhava means. I explained it in #1 above. Also see “Bhava and Jāti – States of Existence and Births Therein.”
