Office 365 external sharing and Azure Active Directory B2B collaboration
External sharing in Office 365 (OneDrive, SharePoint Online, Unified Groups, etc.) and Azure Active Directory (Azure AD) B2B collaboration are technically the same thing. All external sharing (except OneDrive/SharePoint Online), including guests in Office 365 Groups, already uses the Azure AD B2B collaboration invitation APIs for sharing.
OneDrive/SharePoint Online has a separate invitation manager. Support for external sharing in OneDrive/SharePoint Online started before Azure AD developed its support. Over time, OneDrive/SharePoint Online external sharing has accrued several features and many millions of users who use the product's in-built sharing pattern. However, there are some subtle differences between how OneDrive/SharePoint Online external sharing works and how Azure AD B2B collaboration works:
OneDrive/SharePoint Online adds users to the directory after users have redeemed their invitations. So, before redemption, you don't see the user in Azure AD portal. If another site invites a user in the meantime, a new invitation is generated. However, when you use Azure AD B2B collaboration, users are added immediately on invitation so that they show up everywhere.
The redemption experience in OneDrive/SharePoint Online looks different from the experience in Azure AD B2B collaboration. After a user redeems an invitation, the experiences look alike.
Azure AD B2B collaboration invited users can be picked from OneDrive/SharePoint Online sharing dialog boxes. OneDrive/SharePoint Online invited users also show up in Azure AD after they redeem their invitations.
To manage external sharing in OneDrive/SharePoint Online with Azure AD B2B collaboration, set the OneDrive/SharePoint Online external sharing setting to Only allow sharing with external users already in the directory. Users can go to externally shared sites and pick from external collaborators that the admin has added. The admin can add the external collaborators through the B2B collaboration invitation APIs.