How does the secondary database know that it has caught up the end of log on the primary database?

From the URL below about "How Synchronization Works on a Secondary Replica" , It describes that  "Once the hardened log on the secondary database has caught up the end of log on the primary database, the state of the secondary database is set to SYNCHRONIZED. "

My question is how does the secondary database know that it has caught up the end of log on the primary database?

Suppose there are five transaction log records in primary database, and three of them transfered to the secondary database, how does the secondary database confirm that there are two remains log records haven't arrive? Even if all of the five records transfered to the secondary database, how does it know there is no log records left in the primary database ?

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/availability-modes-always-on-availability-groups?view=sql-server-2017

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