Hi anonymous user,
I understand that you are looking to merge Azure tenants into one and also list the service principal and application identities before migration.
To list service principals in a tenant, you can use:
Azure AD Powershell:
Get-AzureADServicePrincipal
Or Azure RM Powershell:
Get-AzureRmADServicePrincipal
To consolidate the tenants, you will need to manually add the new user objects in the Azure AD and provide the necessary permissions. There is no out-of-the box tool in Azure or Microsoft 365 that does this, so you will need to re-create the users and resources.
Merging tenants is a relatively complex task and it is difficult to provide precise guidance for this, especially if you have larger tenants or have services outside of Azure AD such as Sharepoint and mailboxes. You can move entire subscriptions between tenants, but if you do that you will lose all of your RBAC assignments and not every resource can handle something like that. You will most likely need to recreate many of your resources.
Depending on your needs, you can also look into Azure Lighthouse, which enables logical projection of resources from one tenant to another tenant. It does not, however, physically move data or resources.
Another option would be to use the ADMT migration tool to consolidate the on-premises Active Directories and then synchronize them.
Additional resources:
Cross-tenant management experiences
Merge/move two ADs and one Office 365 tenant to a new AD
Microsoft 365 tenant-to-tenant migration
Retrieve all service principals from the directory
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