I have found the answer.
Say there are 3 FDs
created: 1 Timer, 1 IPC & 1 MQTT.
When these are created then subsequent interfaces are assigned to each interface FD which is inturn added to Poll FD.
Any event happening on each interface ID will be read by Poll FD and for which interface it happened will be known.
That's how it will be known for which interface is the event meant. Its not the case that any external module assigns the epoll interface ID, its rather handled internally.