TL;DR: PatientService.savePatient() → mpi-client AOP advice (PatientSynchronizationAdvice) → async PatientUpdateWorker → MpiClientService.exportPatient() → FHIR conditional-update (upsert) POST to OpenCR on the SEDISH source-key.
Repo (canonical): IsantePlus/openmrs-module-mpi-client (main). Registration trigger lives in charess-org/iSantePlus (registration module).
registration/api/src/main/java/org/openmrs/module/registration/api/impl/RegistrationServiceImpl.java (~line 729)