OK, that's got all my toys updated to the new versions. Two Apple Silicon Macs on macOS 15; five HomePods on HomePodOS 18; two iPads on iPadOS 18, Apple TV on tvOS 18 and the iPhone on iOS 18. There was no update for the Apple Watch.
The HomePods consist of a stereo pair of Minis, a stereo pair of 2nd gen big ones and a single 1st gen big one. A single HomePod updates in the same amount of time as a stereo pair.