macOS Ventura already ships a software AV1 decoder and AV1 hardware decoder plugin. But the software decoder is not enabled by default, and I don't know if the hardware decoder is already used on some hardware or not to decode AVIF at least. 13.4 creates preview for AV1 .mp4 files in Finder now.
I did not test macOS 14 yet, but there is a new framework to create format readers and video decoders plugins:
https://developer.apple.com/documentation/mediaextension
Probably won't be needed for AV1, but it can be used for other formats.