To back up a web app, you need to configure a custom backup that specifies a storage account and a container as the target for the backup1. The storage account must be in the same subscription as the web app, and the container must be accessible by the web app2. The backup size is limited to 10 GB, and the backup frequency can be configured to minimize costs.
According to the table, storage1 is the only storage account that meets these requirements. Storage1 is in the same subscription and region as the web app, and it is a general-purpose v2 account that supports custom backups. Storage2 and storage3 are in a different region than the web app, which may incur additional costs for data transfer. Storage4 is a FilesStorage account, which does not support custom backups.
Therefore, you should use storage1 as the target for the backup of your web app. To configure a custom backup, you can follow these steps:
In your app management page in the Azure portal, in the left menu, select Backups.
At the top of the Backups page, select Configure custom backups.
In Storage account, select storage1. Do the same with Container.
Specify the backup frequency, retention period, and database settings as needed.
Click Configure.
At the top of the Backups page, select Backup Now.

Box 1: Group 1 only
First rule applies
Box 2: Group1 and Group2 only
Both membership rules apply.
References: https://docs.microsoft.com/en-us/sccm/core/clients/manage/collections/create-collections

The virtual networks can be in the same or different regions, and from the same or different subscriptions.
When connecting VNets from different subscriptions, the subscriptions do not need to be associated with the same Active Directory tenant.
Configuring a VNet-to-VNet connection is a good way to easily connect VNets. Connecting a virtual network to another virtual network using the VNet-to-VNet connection type (VNet2VNet) is similar to creating a Site-to-Site IPsec connection to an on-premises location. Both connectivity types use a VPN gateway to provide a secure tunnel using IPsec/IKE, and both function the same way when communicating.
The local network gateway for each VNet treats the other VNet as a local site. This lets you specify additional address space for the local network gateway in order to route traffic.
Reference:
https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-howto-vnet-vnet-resource-manager-portal

Reference:
https://docs.microsoft.com/en-us/azure/backup/backup-azure-arm-restore-vms
https://docs.microsoft.com/en-us/azure/backup/backup-azure-restore-files-from-vm#system-requirements

The Free Tier provides 60 CPU minutes / day. This explains why App1 is stops. The Basic tier has no such cap.
References:
https://azure.microsoft.com/en-us/pricing/details/app-service/windows/