what is the wood species of the ceiling plywood and how did you hang them? Thank you!

The wood is maple - See this thread for additional details. https://www.houzz.com/discussions/wonderful-blending-of-aged-exterior-and-clean-modern-interior-dsvw-vd~3091575

Out of curiosity, have you ever used Baltic birch? If not, is there a reason maple is preferred?

Baltic birch is an option too. At the time I believe that maple was readily available.