Just to be clear, integrations and addons are different things. Whatever you found under the integrations Integrations - Home Assistant (home-assistant.io) page, are all part of the core - you’d get all of those regardless how you install your HA.
Add-ons, on the other hand, are services that are “related” to your home setup, and only applicable to HAOS install method. If you want a different database? That’s available as an add-on. Plex? That’s available as an add-on. You want Node-Red for automation instead of the built-in automation editor/engine? That’s available as an add-on. You want to do Cloudflared so that to connect to your HA services from outside? That’s available as an add-on.
So these are official addons: home-assistant/addons: Docker add-ons for Home Assistant (github.com)
And these are community addons: Home Assistant Community Add-ons (addons.community)
And then there are other add-ons that were developed independently, for example Google Drive Backup, or Zigbee2MQTT.
In the background, addons are containers also. They are just packaged specifically for HAOS, so that to smooth out the learning curve for users who choose HAOS.
== An example ==
One might ask: wait a minute, what if I go container-based installation, and want to do Plex? Couldn’t I just install Plex server somewhere else myself?
The answer is sure, of course you could, and that’s the point: you very likely could find equivalents of those HAOS add-ons available somewhere else in a different format, and you could totally do that yourselves if you want to.
Now, what if I want to integrate my HA to my Plex server (that is in another metal box)?
Sure you could do that also. Search “plex” under the aforementioned integrations page, and you will find how you could connect HA and Plex together, and build your own interesting automations. 