When a change is to be made, the data page is fetched from the database data file into the buffer and changes are made to the buffer page and are also written to the transaction log. Write ahead logging. In Simple recovery mode the changes from the buffer are applied to the database data pages at checkpoint. The checkpoint flushes the records from the transaction log. Hence transaction log backups cannot be taken in this mode. Similarly for full/bulk recovery models when a full backup hasn't yet been taken.
Question: How and when does the changes to the data page get written to the database data file in the full/bulk recovery model once the 1st full backup has been taken?