What do you mean with “Operating System”?

If I go to

https://www.home-assistant.io/

and click on “Installation”, ignoring the custom Home Assistant hardware, the first relevant section is “DIY with Raspberry Pi” whose tutorial has a section “Install Home Assistant Operating System”.

The second relevant section of the Installation page is “Install on other hardware” with a paragraph whose second sentence is “The Home Assistant Operating System allows you to install Home Assistant on these devices even if you have little to no Linux experience.”