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

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.

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

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.