Integrations in HACS (or custom_components as they are often referred to) are community-built integrations.
Special notes about Integrations
When you download, update or remove an integration in HACS, you need to restart Home Assistant.
You may need to clear your browser cache before you see it in the "+" list.
Note that this only applies for the initial download of that Integration, if you update it, you still need to restart Home Assistant before it loads the new files.
Integrations download location
Any Integration you download with HACS is stored in
custom_components/ in your Home Assistant configuration directory.