Hybrid Azure AD is a combination of on-premises Active Directory and Azure AD. To join a hybrid Azure AD, you need to open the portal, sign in, and go to the hybrid account settings. If the domain controller is listed in your on-premises Active Directory, then the hybrid Azure AD is already joined.
If the domain controller is not listed, then you need to add the domain controller to your on-premises Active Directory. Once the domain controller is added, the hybrid Azure AD is joined.
If you want to verify that the hybrid Azure AD is joined, you can use the following command:
Get-ADDomainController -Filter “Name -like ‘*hybrid*'”
The command will return a list of domain controllers that are configured as hybrid Azure AD.