Explanation:
Step 1: Create a Recovery Services vault.
Immutable vault for Azure Backup
Immutable vault can help you protect your backup data by blocking any operations that could lead to loss of recovery points. Further, you can lock the Immutable vault setting to make it irreversible to prevent any malicious actors from disabling immutability and deleting backups.
Step 2: Enable vault immutability
Step 3: Lock immutability for the vault
Making immutability irreversible
The immutability of a vault is a reversible setting that allows you to disable the immutability (which would allow deletion of backup data) if needed. However, we recommend you, after being satisfied with the impact of immutability, lock the vault to make the Immutable vault settings irreversible, so that any bad actors can't disable it. Therefore, the Immutable vault settings accept following three states.
Reference:
https://learn.microsoft.com/en-us/azure/backup/backup-azure-immutable-vault-concept