How Software Update settings obtains a list of available updates, and the variants that should contain. How mobile asset catalogues are checked, and preparations made to download the update.
softwareupdated
Software update was completely redesigned for Big Sur, using finite state machines to enable reliable, flexible and robust updates to be applied to macOS. These are documented in log entries made during an update.
How macOS 26.2 updated to 26.3, variation in download sizes, how macOS works out free space requirements to proceed with the update, and its sequence of stages run by a finite state machine.
At 18:00 on 16 September, XProtect 5315 was released for all Macs. Sequoia systems were unable to update for 24 hours, and Tahoe silently updated when it felt like it. How come?
There should be an update available, but you can’t find it, or it fails to install, or you have problems with a Content Caching Server. How to tackle these.
Opening the settings runs a partial check for updates, but omits important updates, and almost immediately loses its help button.
The Eclectic Light Company 