Short answer
HubSpot's KB: the app attaches and organizes OneDrive and SharePoint files across records and workflows. A Super Admin or user with Customize record page layout permission must add the card to records. You need access to the files you link, and workflow actions can create and organize files.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
You can connect SharePoint or OneDrive first and add the other later.