Delve shows you documents shared in SharePoint Online and OneDrive for Business. If you’re not seeing a lot of content in Delve, chances are it’s because you haven’t started storing all your personal files in OneDrive for Business, and your shared files in SharePoint Online Team Sites.
You will only see another person's document's in Delve if you have access to it in either Team Sites or OneDrive for Business.
The steps on this can be found the “One Drive for Business” Document
To use Delve, type a keyword or title of a document you are looking for and the UI will update automatically to show you the most recently accessed documents and the owner of the document