OAuth apps
Saving vendor OAuth apps (superuser only).
One connected Google account covers Gmail and Sheets: the assistant can read spreadsheets and send mail as the logged-in user. The assistant never holds the user's real access token; see Google token flow for how.
HubSpot
One connected HubSpot account per harnesi user lets the assistant read and, after your approval, write CRM records. See Vendor APIs for how the tools reach HubSpot.