One of the fundamental components of setting up Office 365 is installing Azure AD Connect.

This tool is used to connect your on-premises Active Directory to Azure AD. It works by synchronizing a copy of objects in the directory, such as users, groups, contacts and devices from Active Directory to Azure AD every 30 minutes. […]
https://officerambo.com/how-to-quickly-install-and-configure-azure-ad-connect/