Details of some of what is going on are in the links I recently added to macOS, diskarbitrationd.
As well as patching system vulnerabilities, much of the work on macOS 15 seems to be fixing bugs in macOS which have long been known, especially in Intel code. It occurs to me that more fully debugging macOS (Intel) may be background which enables M-series Apple Silicon advances.
Years ago iOS was created as a single-user fork of the OS X/macOS variant of BSD UNIX, with security, stability, reduced code size, and battery-saving reduced compute overhead being driving virtues. Complementing these software advances, Apple moved iOS onto the newly created Apple Silicon A series. The complex A series SOC (System On a Chip) was the incarnation in silicon of iOS.
Encoding in silicon seriously hardwires software. For this to work, economically as well as by other criteria, the software model which becomes siliconized must be as close to free of bugs as possible. The success of Apple’s A series processors, and the products they enable, reflects the stability and lack of error in the simplified iOS branch of macOS on which this Apple silicon is based.
Similarly, the software basis for Apple’s M series processors is the much more complex multi-user, multi-processor, multi-much more many other things, macOS. As Apple Computer became Apple (“the iPhone company”), many of us were dismayed as more and more bugs accumulated in the neglected macOS. When the iPhone company returned to encode the full UNIX macOS system in M-series silicon, its SOC expertise developed creating the A series based on iOS indicated bugs in macOS on which the M chips would be based would be a serious liability. Software work-arounds for bugs immortalized by encoding in silicon reduce compute efficiency as well as economic viability.
I think Apple’s recent (last several years?) focus on debugging macOS is driven by the newly recognized need for software model which is as bug-free as possible on which to base M-series silicon. Debugging and hardening against intrusion can be done much more efficiently and economically in software than in silicon. Though requiring considerable resources, and critically important for the future of Apple, none of this work is suitable for promotion in glossy retail advertisements. I suspect this explains at least a part of why Apple is quiet with its security, and bug fix, updates these days.