As per MongoDB BOL Here
In order to maintain up-to-date copies of the shared data set, secondary members of a replica set
sync or replicate data from other members. MongoDB uses two forms of data synchronization:
Initial sync copies all the data from one member of the replica set to another member.
Even the initial sync to populate new members with the full data set, and replication to apply ongoing changes to the entire data set.
How the Process happened in Sync
When you perform an initial sync, MongoDB:
Clones all databases except the
local database. To clone, the
mongod scans every collection in each source database and inserts all data into its own copies of these collections.
Changed in version 3.4: Initial sync builds all collection indexes as the documents are copied for each collection. In earlier versions of MongoDB, only the _id indexes are built during this stage.
Changed in version 3.4: Initial sync pulls newly added oplog records during the data copy. Ensure that the target member has enough disk space in the local database to temporarily store these oplog records for the duration of this data copy stage.
Applies all changes to the data set. Using the oplog from the source, the
mongod updates its data set to reflect the current state of the replica set.
When the initial sync finishes, the member transitions from
Secondary members replicate data continuously after the initial sync. Secondary members copy the
oplog from their sync from source and apply these operations in an asynchronous process.
Secondaries may automatically change their sync from source as needed based on changes in the ping time and state of other members’ replication.
Changed in version 3.2: MongoDB 3.2 replica set members with
1 vote cannot sync from members with
Secondaries avoid syncing from
delayed members and
If a secondary member has
members[n].buildIndexes set to true, it can only sync from other members where
buildIndexes is true. Members where
buildIndexes is false can sync from any other member, barring other sync restrictions.
buildIndexes is true by default.
For Your Further Ref Replica Set Oplog and Replica Set Deployment Architectures