That's an interesting point. As I recall (unfortunately it's been a while). There was tension at first who would be top dog. SharePoint or OneDrive, and OneDrive won, so SharePoint Online is actually mapped over OneDrive. MS could do that because ironically even though SharePoint On-prem had been entirely SQL Server underneath, SharePoint On-prem devs generally never wrote a line of SQL. They interacted with the SharePoint Object model and that was basically mapped over OneDrive. What OneDrive is underneath I don't think MS had to be as forthcoming about because of it's cloud-based nature. I was on my way out by that point though.