Hi,
These might help:
Multiple forests, single Azure AD tenant
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies#multiple-forests-single-azure-ad-tenant
What is Azure AD Connect cloud provisioning?
https://learn.microsoft.com/en-us/azure/active-directory/cloud-provisioning/what-is-cloud-provisioning
Best regards,
Leon