Replica set data sync questions

Hello i have a question to the replica set.

When i change during data input the replicat set primary node to another one wiht rs.stepDown(). Why i don’t see no more data input is coming into the database. I also see no more data input when i change the node back.

Is there only a specific time when the sync occurs and how i can i trigger it manually ?

Or update mongo the data if only a specific amount is comming in and not for a single document?

Because when i search after update of1 document i can’t find it but whe i insert thousend of doucments i can find it.

Is the data all in the oplog ?

I have a lot of data in the oplog can i delete this if the data was only for testing?

Because i can not see what is into the oplog when it is so big?