InmateAid maintains a database of about 1.5 million inmates drawn from publicly available correctional records across the country. That database gets updated regularly through a data service that pulls current inmate designations from state and federal correctional systems, which is why your husband was already in the system when you searched for him even though you had never used the service before.
The information that appears in those pre-existing profiles comes from the same public records that state departments of corrections and the Bureau of Prisons make available through their own inmate locator tools. Name, facility, inmate identification number, and basic sentence information are all publicly accessible data points that InmateAid incorporates into its database to make the setup process faster and easier for new users.
Finding your husband already listed is actually a good thing. It means the profile information is likely accurate and you can start using InmateAid's services immediately without having to manually enter facility details or verify address information on your own. The profile is already synced to his current location, which means letters, photos, and any other services route to the right place from the start.
If you ever cannot find an inmate in the system, which happens with smaller county jails or facilities that do not share data publicly, you can create a profile manually by entering the facility information yourself. That manual profile works just as reliably as a pre-populated one for all of InmateAid's services.