As a developer, I can easily find out how the FHIR standard structures are mapped onto HAPI implementation classes – either by reading the documentation or by reviewing the code. For example, I can see that the R4 Patient, which has a resource type of Patient and the profile URL http://hl7.org/fhir/StructureDefinition/Patient is implemented as org.hl7.fhir.r4.model.Patient – not least because this exact information is contained in the @ResourceDef annotation of the class.