The Registro.it or .it Registry, is the authority responsible for the allocation of .it Domain Names.
It could be considered as the registry office of .it Domain Names, as it establishes the regulations and policies of Domain name allocation, the management of databases and primary Nameservers of the various TLDs.
The Registration of Domain Names cannot be performed directly through the Registro.it.
The Registrant, or rather the individual, company, authority or association requesting the Domain registration and subsequent allocation, needs to go through a Provider or Registrar:
A service provider accredited by a Registry that manages the reservation of Domain Names.
The Registrar acts as a mediator with the Registry for all the Domain Name Registrations. Registrars also provide other post-registration Domain Name related services. Aruba is a .it Domain accredited Registrar.
In order to see with which Registrar domains are registered, simply perform a whois search.