https://wccftech.com/apples-homepod-is-now-based-on-tvos-heres-what-you-need-to-know/

Apple's smart speaker was based on a cut down version of iOS which the company called HomePod software. Now, there have been several major changes made which made their debut in the latest HomePod 13.4 update. Now, the HomePod is based on tvOS instead of iOS.

tvOS is based on iOS as well but since it runs on the Apple TV which is always connected to a power source, the changes made to the base make more sense. Take note that there are no major changes on the front-end which means the user experience will remain the same after the update.