What happens on checkpoint?

A checkpoint writes the current in-memory modified pages (known as dirty pages) and transaction log information from memory to disk and, also records the information in the transaction log. ... For more information, see Change the Target Recovery Time of a Database (SQL Server)

https://docs.microsoft.com/en-us/sql/relational-databases/logs/database-checkpoints-sql-server?view=sql-server-ver15 

https://sqlbak.com/academy/checkpoint