SharePoint and Office 365 Official Documentation

Office 365

Office 365 General
https://docs.microsoft.com/en-us/office365/

Azure AD
https://docs.microsoft.com/en-us/azure/active-directory/

Flow
https://docs.microsoft.com/en-us/flow/

OneDrive
https://docs.microsoft.com/en-us/onedrive/

PowerApps
https://docs.microsoft.com/en-us/powerapps/

SharePoint
https://docs.microsoft.com/en-us/sharepoint/

Teams
https://docs.microsoft.com/en-us/MicrosoftTeams/Microsoft-Teams

Reference

Azure

Azure General Information
https://docs.microsoft.com/en-us/azure/
Domain Services
https://docs.microsoft.com/en-us/azure/active-directory-domain-services/