The process of backing up Always On availability groups is the same as for other SQL databases. See Backups for SQL Server.
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.
You can run the following backup types on the primary and secondary replica:
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Full and differential backups: These backups always run on the primary replica.
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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.