How do I open Nikon Z6iii RAW files in NEF format on MacBook Pro without additional software?

Apple does not support RAW files from the Nikon Z6iii in either Sonoma or Sequoia (yet). The Z6, and Z6ii are supported, but the RAW format for the Z6iii requires data from Nikon. Apple's camera RAW support is not retroactive to earlier operating systems to at some point in Sequoia or macOS 16, Apple may/may not implement RAW support for the Nikon Z6iii camera.

In the meantime, you can use the current Adobe DNG Converter (17.2, 2.6 GB, minimum Ventura) to convert the NEF images into DNG format which is supported in the Finder, Quick Look, and Apple's Preview.

Alternatively, you can use the free XnView MP tool to directly view the Z6iii RAW images.

Unless you use Apple's Image Capture to transfer the image from the Nikon — the Finder will request the operating system (UNIX) to use the System time for that image creation date. Same story if you copied the file from external media. Image Capture tells the Finder to use the EXIF DateTimeOriginal stamp for the file creation date.

The Adobe DNG converter v17.2 converted a sample 19.6 MB Z6iii NEF RAW image to a 26.3 MB DNG image. Certainly did not double the resulting size. DNG Converter does not use the EXIF DateTimeOriginal stamp for the new DNG creation date. That's on Adobe, not Apple.

When I open the Z6iii sample NEF image in Affinity Photo 2 v2.6.0, the image compares faithfully with the converted DNG image viewed in Apple's Quick Look. It is not Apple's fault if the Adobe DNG Converter does not use the EXIF DateTimeOriginal for the converted DNG creation date. No one outside of the Adobe DNG Converter product team can qualify the differences between the RAW and converted DNG images.