SAP S/4HANA integrated with third party HR system

Hello everyone,

I am working on a project where the client required an integration of SAP S/4HANA with an HR solution (third-party).

How the integration works ?

What does the integration required ?

The data flow between the 2 systems has to be 2 directions ?

Do we still need SAP HCM active to integrate the Employee master Data with The third party HR sol ?