Standard way
You probably know that you can get the tenant's root site by calling
GET https://graph.microsoft.com/v1.0/sites/root
GET https://graph.microsoft.com/v1.0/sites/tenant_name.sharepoint.com
And to get the drive aka document library, simply add /drive to the url
GET https://graph.microsoft.com/v1.0/sites/root/drive
GET https://graph.microsoft.com/v1.0/sites/tenant_name.sharepoint.com/drive
You've probably used it at least once in your life.
But what if there is another way to access the document library of the default root site?
Undocumented way
I didn't find any notes in the doc about the second way to access the default drive, but here it is
GET https://graph.microsoft.com/v1.0/drive
Nice, isn't it. Easy to use, easy to remember.
Permissions
In both cases, you need Files.Read.All permission.
