You can speed up the learning process if you search for UStaticMesh within fbx importer source files.
Bulk of Fbx code (*.cpp files) is located at “UnrealEngine\Engine\Source\Editor\UnrealEd\Private\Fbx”, where “UnrealEngine” is source code root folder. Looks like “FbxStaticMeshImport.cpp” should be of interest.
Basically, ignore anything that is not interacting with UStaticMesh and see which interaface methods/variables are available.