Partially:

https://github.com/apple-oss-distributions/distribution-macO...

https://github.com/apple-oss-distributions/distribution-iOS

I'm not sure how it all fits together but people have even made an open source distribution of the base of darwin, the underlying OS:

https://github.com/PureDarwin/PureDarwin

Apple's FOSS releases are purely command-line userland tools and their kernel, all the frameworks and servers[0] that make the UI work like you'd expect are 100% proprietary.

FWIW Apple has also been on a decades-long track of purging GPL-licensed code from macOS and replacing it with either permissively-licensed or proprietary equivalents. So they're obligated to release even less than they used to.

[0] AppKit/WindowServer for MacOS and UIKit/Springboard/Backboard for everything else