Apple has just released iOS 11.4.1, HomePod 11.4.1, tvOS 11.4.1 and watchOS 4.3.2 to all eligible devices. There is detailed release note only for the iOS update, which mentions the USB Restricted Mode. The new feature does not allow third parties to break into the phone without the owners agreement, effectively tightening security even more.
All four updates count as maintenance releases, even the iOS one. It can be installed over iTunes or OTA via Software Update feature. The HomePod devices should switch to the new version automatically.
The watchOS 4.3.2 needs you to load the Watch app on a paired iPhone and then go to General, then Software Update. Make sure you have at least 50% charge in the watch, and it is in Bluetooth range. Apple TV and Apple TV 4K can get tvOS 11.4.1 via the Settings app, but you mightve already received it if you enabled automatic updates.
The lack of new features is reserved for the next major version of the iOS, tvOS, and watchOS that should be introduced in September along with the 2018 iPhone. All of them are currently in beta, with Apple releasing iOS 12 public beta 2 in Friday.