I think you need to ask Microsoft how it backs up and recovers its software as a service offerings. That questions seems perhaps off-topic or at least unanswerable here as this site isn't Microsoft.
Determining the the necessary policies for an on-premises SharePoint farm for backup and recovery options to eliminate data loss also seems perhaps off-topic or at least too large a question to be a good one for this format. Perhaps some valuable insight into host and database administration can be had on the Server Fault and Database Administrators SEs.
What troubles me though in conclusion is that if the solution is "critical" and you have "millions of users" and are hosting it as an online service already...ought not have you ascertained the answers to these questions already if you "want 0% data loss" (whatever that means, but it sounds lofty)?