Ask HN: Payroll Sync, how does it work?

Does anyone know how payroll sync works. Do companies like Zenefits build interfaces with each payroll provider or is there a platform which aggregates across payroll providers?

https://help.zenefits.com/Payroll/Getting_Started_With_Payroll_Sync/Learn_the_Basics_of_Payroll_Sync/04-Payroll_Providers_That_Sync_With_Zenefits