From The Apple Wiki

audioOS is the operating system used by the HomePod and HomePod Mini. It is a forked version of tvOS as of 13.4 (prior to 13.4, it was a forked version of iOS).

The first version of audioOS is marketed as 11.0. An OTA update was released before the HomePod's launch as 11.0.2. OTAs with broadened firmware requirements are prefixed with "9.9." (example:

The shell for audioOS is called by "SoundBoard" (instead of SpringBoard), although it has no user interface. Most frameworks and applications are replaced with HomePod equivalents that are prefixed with "Air."