With Azure MARS, your local files are backed up to what's called a Recovery Services vault in Azure. This works whether you're backing up directly from your on-premises Windows machines or from Azure VMs. To see how much space you're using for your backups, you'll use something called Azure Backup Reports. These reports show you how much data you're backing up and how much storage you're using in the vault. Azure lets you set up alerts if you're getting close to your storage limit. This way, you can keep track of your storage usage without constantly checking it manually.
https://learn.microsoft.com/en-us/azure/backup/install-mars-agent
https://learn.microsoft.com/en-us/azure/backup/backup-azure-manage-mars