The fixing of a bug at Apple is the easy and quick part. It's the submission process from then until it gets released as part of an OS update that is the ridiculously long (and too often difficult) part.
The submission process is pretty trivial too - as long as your code gets a PR review, and is given the green light to be merged into trunk (which is the 99% case, even if there are PR comments to address), it's going to be in the next daily build.
The releases are the things that are few and far between - generally though, a nominated daily-build (based on the pre-determined release schedule) is triaged and tested by QA and engineering for a while before release, and then ... it's out there...
...Unless something goes unexpectedly wrong with the nomination, anyway. That's pretty rare because builds are constantly being made, regressions identified, and new bugs discovered and earmarked as "must-fix" (or whatever) on a daily basis. B&I have a fairly good feel for how things are going at any given time.
It's really just timing. If you can squeeze another fix in before the cutoff deadline for the nominated build, you're in. If not, you wait until the next one, which can be a while...
Yeah, that was not at all my experience in CoreOS/SWE, where we would sometimes/often have to wait weeks for submissions to turn around in B&I to become part of "daily" builds. Glad you don't have to put up with the same ridiculous crap process.
It would have to be a very serious security bug. Even then, unless they've totally upended their software development workflows in the past couple of years, the Apple I knew extremely well from the inside couldn't turn around a software fix this quickly, from PR to OS release, even if its existence depended on it. There's simply too much bureaucracy and process around submitting anything, no matter how vital.
At a very low level, it is. I know the individual that made a "diagnostic" for the floppy drive while working as a tech on the Apple I and Apple II designs which caused the drive to whine in patterns that were distinctly ... orgasmic.
The fruit company still has an internal culture, especially in hardware-focused teams, with a relentless focus on shipping products followed by iterative refinement.