First of all, very good question! Really appreciate you having want to understand in such deep detail.
Now, your question,
It behaves as was exaplained in lecture, when a secondary comes back up after some delay, it will sync up with the oplog from primary. And oplog is not so big like it will not have write operations, it will have the text like, it performed a write operation and then write operations are performed as soon after that.
Example: I performed write operation:
The oplog entry will be like “it performed a write”.
The oplog size depends on your system and requirements, like you don’t need large oplog size if you have more read operations than write.
And you can always reconfigure the oplog size.
And as @steevej-1495 explained, Primary oplog size should be increased if you want to make sure that no write operation should be lost.
Hope it helps!
Keep learning, Kanika