HACS installs community integrations whereas addons are like external programs that hook in HA. You can do the same thing with HA in Docker by installing the addon containers separately and then hooking them in manually but HA OS makes it much simpler.

For example I’m running the Mosquitto broker, Z2M, a Visual Studio Code server, diyhue, and Music Assistant as addons.

Docco page about it is here: www.home-assistant.io/addons/