Hi Naga,
the user information (first name, last name, manager, etc.) is picked from the User Detail Data Source. So if you have ECC defined with SU01 then GRC picks the information from ECC via SU01.
Requestors should exist at least in the GRC system to create the access request. In most scenarios users exist in both system (GRC and backend) as most of the time the requestor is a key user who also works in the ECC.
For some reasons it can be that users only exist in GRC. Like managers who only approve access reuqests or IT personnel who doesn't work with ECC.
Considering all in the end it depends on your requirement and how you set up your system and processes. I can only share my approach and how I think it is correct. But as I don't know your business and processes it is very hard to say that's right or wrong.
Hope it helps.
Regards,
Alessandro