You can validate web app changes in a staging deployment slot before swapping it with the production slot. Deploying an app to a slot first and swapping it into production makes sure that all instances of the slot are warmed up before being swapped into production. This eliminates downtime when you deploy your app. The traffic redirection is seamless, and no requests are dropped because of swap operations. You can automate this entire workflow by configuring auto swap when pre-swap validation isn't needed.
After the swap you can deploy the App1 update to webapp1-test, and then test the update. If the changes swapped into the production slot aren't as per your expectation then you can perform the same swap immediately to get your "last known good site" back.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/deploy-staging-slots

Only a global administrator can add users to this tenant.
Reference:
https://docs.microsoft.com/en-us/azure/devops/organizations/accounts/add-users-to-azure-ad

You can find unused disks in the Azure Storage Explorer console.Once you drill down to the Blob containers under a storage account, you can see the lease state of the residing VHD (the lease state determines if the VHD is being used by any resource) and the VM to which it is leased out. If you find that the lease state and the VM fields are blank, it means that the VHD in question is unused. The screenshot below shows two active VHDs being used by VMs as data and OS disks. The name of the VM and lease state are shown in the "VM Name" and "Lease State" columns, respectively.

Reference:
https://cloud.netapp.com/blog/reduce-azure-storage-costs

Reference:
https://medium.com/azure-architects/exploring-azure-private-dns-be65de08f780
https://simpledns.plus/help/dns-record-types