You back up Always On availability group instances the same way that you back up other SQL subclients.
For information about backups of SQL subclients, see SQL Backup Operations.
When you start the backup, the master process retrieves the backup information from the availability group. All activities of the master process are logged in the SQLBackupmaster.log file.
Backup jobs run on the primary or secondary replica as follows:
Full and differential backups: These backups always run on the primary replica.
Transaction log backups: If the primary replica is set as the preferred replica for running backups, then transaction log backups run on the primary replica. Otherwise, the transaction log backups run on the secondary replica that has the highest backup priority.