Typically when working with App Roles in Azure Active Directory for a single application registration or service principal and then self consuming that app role as an Application API Permission you would see in the Enterprise Application > Users and Groups blade that service principals are added.
Every now and then a question comes up on assign service principals (application registrations) to other service principals (application registrations) without creating app roles. Is that possible?
The answer is YES! It is possible.
Here is how: