Raw files of Nikon Z9 not working

The is no RAW support for the Nikon Z9 ibuilt into macOS Big Sur, see the list of supported camera models: Supported digital camera RAW formats - Apple Support

Keep using the Nikon Software to develop your RAW files and to convert them to a format that you can use in other apps.

The Z9 is now supported on macOS Sonoma, but only the lossless RAW format (Digital camera RAW formats supported by iOS 17, iPadOS 17 and macOS Sonoma – Apple Support (UK)). You will be able to work directly with the RAW files, once you get yourself a newer Mac Model, that can be outdated the macOS 14 Sonoma, but even then you could only use the lossless format, not the compressed version.

Hold down the Option key and go to the  menu and select System Information.

When the report appears click on Raw Support in the Sidebar. Then check to see if the Nikon Z9 is supported.

That looks like a good workflow. The software provided by the manufacturer should give you the best possible RAW development like to use it, if you . For my Lumix FZ28 I had to fall back on the Adobe DNG converter to convert the RAW files to DNG, as I did not like SILKYPIX Raw editor that came with the Lumix camera. The user interface has not been very intuitive.

Even though Photos recognizes my Z5's NEF files, I do most of the work with JPEGs. My camera saves both NEF and jpeg, on separate SD cards. My jpegs are pretty much bland--very little in-camera enhancements. I load the jpegs into Photos and apply enough editing to choose the best pictures-- and then I think about what the RAW information might do for me. For the few that I think might benefit, I go back to the NEF files and work with them in NX Studio. I do what that program does well, and then I produce a jpeg100 image and import that into Photos. In Photos, I continue doing what Photos does, for me, better than NX Studio.

I don't have a lot of experience at this RAW thing, yet (léonie has way better understanding of this) but, so far, that's what's working for me...