When you implement Oracle Identity Management with Oracle EBusiness Suite, then first the records get created in Human Resources. This is because the Human Resource team is the first team to know that someone is hiring.
With Identity Management in place and integrated, the HR records in PER_ALL_PEOPLE_F get picked up with Oracle Identity Management reconciler process, that sucks our records from per_all_people_f and puts into USR table of Oracle Identity Management database. From OIM, the records get synchronised into Oracle Internet Directory, and from there they go into FND_USER.
In typical cases you will find the flow to be
HRMS System -->Central Data Warehouse[if exists]->OIM->AD->OID->FND_USER
If HR System is Oracle EBS, then the flow will be
PER_ALL_PEOPLE_F -->Central Data Warehouse->OIM->AD->OID->FND_USER
If there is no Active Directory, lets assume
PER_ALL_PEOPLE_F -->Central Data Warehouse->OIM->OID->FND_USER
The two diagrams below show how these links are created between the different layers during Oracle Identity Management implementation with Oracle EBusiness Suite.