ArcGIS Runtime SDK for iOS cannot open file geodatabases (you would need Local Server for that, which is only available on .NET, Java, and Qt when deploying to Windows or Linux).
I would suggest using ArcGIS Pro to create a Mobile Geodatabase, which can be opened using the AGSGeodatabase class.
You could also conceivably use a GeoPackage but a) that is more limited than a Mobile Geodatabase in terms of the schema it supports (attachments, related records, domains, etc.) and b) it would require a Standard Runtime license.
