How OneDrive safeguards your data in the cloud, including the Personal Vault feature:

https://support.office.com/en-us/article/How-OneDrive-safeguards-your-data-in-the-cloud-23c6ea94-3608-48d7-8bf0-80e142edd1e1


https://support.office.com/en-us/article/be-safe-online-ce495131-eb83-4dc7-acea-6304a220372e?wt.mc_id=jtbd_safeonline&WT.mc_id=eml_CXM_EN-US_C_EmergencyPrep_ItssecurewithOneDrive



MFA (Multi-factor authentication) with external users on Onedrive or Sharepoint specific shares:

https://blogs.technet.microsoft.com/kekirkpa/2017/10/13/enforcing-multi-factor-authentication-for-external-users-on-sharepoint-online/

Azure AD Premium needed to setup MFA with external users:

https://www.royaldiscount.com/blog/azure-ad-premium-plan-comparison/


Sharing with External users:

https://www.windowscentral.com/share-files-external-users-through-onedrive-business-and-sharepoint


Sharing with email verification code process:

https://mspoweruser.com/microsoft-rolls-secure-external-sharing-experience-onedrive-sharepoint/