Explanation

Step 1: Run the following commands
Install the container runtime.
Azure IoT Edge relies on an OCI-compatible container runtime. For production scenarios, we recommended that you use the Moby-based engine provided below. The Moby engine is the only container engine officially supported with Azure IoT Edge. Docker CE/EE container images are compatible with the Moby runtime.
Install the Moby engine.
sudo apt-get install moby-engine
Install the Moby command-line interface (CLI). The CLI is useful for development but optional for production deployments.
sudo apt-get install moby-cli
Install the security daemon. The package is installed at /etc/iotedge/.
sudo apt-get install iotedge
Step 2: From Iot Hub,create an IoT Edge device registry entry.
Note: In your IoT Hub in the Azure portal, IoT Edge devices are created and managed separately from IOT devices that are not edge enabled.
* Sign in to the Azure portal and navigate to your IoT hub.
* In the left pane, select IoT Edge from the menu.
* Select Add an IoT Edge device.
* Provide a descriptive device ID. Use the default settings to auto-generate authentication keys and connect the new device to your hub.
* Select Save.
Retrieve the connection string in the Azure portal
1. When you're ready to set up your device, you need the connection string that links your physical device with its identity in the IoT hub.
2. From the IoT Edge page in the portal, click on the device ID from the list of IoT Edge devices.
3. Copy the value of either Primary Connection String or Secondary Connection String.
Step 3: Add the connection string to..
To manually provision a device, you need to provide it with a device connection string that you can create by registering a new device in your IoT hub.
Open the configuration file.
sudo nano /etc/iotedge/config.yaml
Find the provisioning configurations of the file and uncomment the Manual provisioning configuration section. Update the value of device_connection_string with the connection string from your IoT Edge device.
Save and close the file.
After entering the provisioning information in the configuration file, restart the daemon:
sudo systemctl restart iotedge
Reference:
https://docs.microsoft.com/en-us/azure/iot-edge/how-to-install-iot-edge-linux

Explanation
Device twins store device-related information that:
Device and back ends can use to synchronize device conditions and configuration.
The solution back end can use to query and target long-running operations.
Desired properties. Used along with reported properties to synchronize device configuration or conditions. The solution back end can set desired properties, and the device app can read them. The device app can also receive notifications of changes in the desired properties.
Reference:
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-device-twins

Manual failover is a feature of the IoT Hub service that allows customers to failover their hub's operations from a primary region to the corresponding Azure geo-paired region. Manual failover can be done in the event of a regional disaster or an extended service outage. You can also perform a planned failover to test your disaster recovery capabilities, although we recommend using a test IoT hub rather than one running in production.
Reference:
https://docs.microsoft.com/en-us/azure/iot-hub/tutorial-manual-failover

D: Once you create IoT Edge modules with your business logic, you want to deploy them to your devices to operate at the edge.
B: Configure a deployment manifest. A deployment manifest is a JSON document that describes which modules to deploy, how data flows between the modules, and desired properties of the module twins.
You deploy modules to your device by applying the deployment manifest that you configured with the module information.
In the Visual Studio Code explorer view, expand the Azure IoT Hub section, and then expand the Devices node.
To confirm that the device you've chosen is an IoT Edge device, select it to expand the list of modules and verify the presence of $edgeHub and $edgeAgent. Every IoT Edge device includes these two modules.
Select Create Deployment for Single Device.
Navigate to the deployment manifest JSON file that you want to use, and click Select Edge Deployment Manifest.
Reference:
https://docs.microsoft.com/en-us/azure/iot-edge/how-to-deploy-modules-vscode