Architecture: How mongodb replication works and secondary node get data from primary node

Here is the simple diagram which explains how the middle man oplog watches/monitors primary and pull any new operation or changes happened on primary and records them in oplog, which eventually get read by all secondaries to update them selves.

1. insert operation happened in primary
2. Operation got logged in oplog ( which is part of local db – oplog.$main )
3. Secondary reads data from oplog and update them selves.