Will Apple's "PHOTOS" be able to process Nikon's Z9 raw files?

They never comment, RAW support just appears with an update or upgrade to the OS. Some cameras *never* get supported, like my Olympus T5. Nikons and Canons tend to be supported, however.

Digital camera RAW formats supported by iOS 15, iPadOS 15, and macOS Monterey - Apple Support

Probably, but it tends to lag a few months. I would guess with the next version of the OS.

Which is a tell, you will have to be able to run the next OS. If you have an older computer you may be out of luck.

As we can never be sure, if a new camera model will get RAW support, I am waiting with the purchase of an expensive camera, until it appears on the RAW support list.