Getting SFSF Employee Central (EC) talk to SFSF Employee Central Payroll is well covered in the official SAP help pages.
However, another common integration scenario that has merged and is becoming quite popular is rather delegate this task to a SFSF EC digital twin deployed [as a micro-service] on SAP BTP platform and let it talk to S/4HANA ECPayroll instead.
This approach is explained in more details here.
In either case the integration mechanism is OAuth2.0 with the OAuth2SAMLBearerAssertion flow. And precisely, the main complexity with this integration is with the SFSF ECPayroll ABAP side as this requires a rather intermediate/advanced understanding of SAP S/4HANA ABAP OAuth2 configuration steps.
In order to make it easier and more comprehensive, I have designed a 3-step solution blueprint.