Upstream sources enable you to manage all of your product's dependencies in a single feed. We recommend publishing all of the packages for a given product to that product's feed, and managing that product's dependencies from remote feeds in the same feed, via upstream sources. This setup has a few benefits:
* Simplicity: your NuGet.config, .npmrc, or settings.xml contains exactly one feed (your feed).
* Determinism: your feed resolves package requests in order, so rebuilding the same codebase at the same commit or changeset uses the same set of packages
* Provenance: your feed knows the provenance of packages it saved via upstream sources, so you can verify that you're using the original package, not a custom or malicious copy published to your feed
* Peace of mind: packages used via upstream sources are guaranteed to be saved in the feed on first use; if the upstream source is disabled/removed, or the remote feed goes down or deletes a package you depend on, you can continue to develop and build References: https://docs.microsoft.com/en-us/azure/devops/artifacts/concepts/upstream-sources?view=vsts

Explanation

Box 1: Progressive exposure
Continuous Delivery may sequence multiple deployment "rings" for progressive exposure (also known as
"controlling the blast radius"). Progressive exposure groups users who get to try new releases to monitor their experience in "rings." The first deployment ring is often a "canary" used to test new versions in production before a broader rollout. CD automates deployment from one ring to the next and may optionally depend on an approval step, in which a decision maker signs off on the changes electronically. CD may create an auditable record of the approval in order to satisfy regulatory procedures or other control objectives.

Box 2: Feature flags
Feature flags support a customer-first DevOps mindset, to enable (expose) and disable (hide) features in a solution, even before they are complete and ready for release.
Box 3: Blue/green
Blue/green deployments which means that instead of replacing the previous version (here we refer to this version as blue), we bring up the new version (here referred to as the green version) next to the existing version, but not expose it to the actual users right away. On the condition of having successfully validated that the green version works correctly, we will promote this version to the public version by changing the routing configuration without downtime. If something is wrong with the green version we can revert back without users every noticing interruptions.
Reference:
https://docs.microsoft.com/en-us/azure/devops/learn/what-is-continuous-delivery
https://docs.microsoft.com/en-us/azure/devops/migrate/phase-features-with-feature-flags
https://medium.com/@denniszielke/continuous-kubernetes-blue-green-deployments-on-azure-using-nginx-appga

There are two different ways to deploy templates to Azure DevOps Services. Both methods provide the same
results, so choose the one that best fits your workflow.
1. Add a single step to your build pipeline that runs the PowerShell script that's included in the Azure
Resource Group deployment project (Deploy-AzureResourceGroup.ps1). The script copies artifacts and
then deploys the template.
2. Add multiple Azure DevOps Services build steps, each one performing a stage task.
The first option has the advantage of using the same script used by developers in Visual Studio and providing
consistency throughout the lifecycle.
References:
https://docs.microsoft.com/en-us/azure/vs-azure-tools-resource-groups-ci-in-vsts

Slack is a popular team collaboration service that helps teams be more productive by keeping all communications in one place and easily searchable from virtually anywhere. All your messages, your files, and everything from Twitter, Dropbox, Google Docs, Azure DevOps, and more all together. Slack also has fully native apps for iOS and Android to give you the full functionality of Slack wherever you go.
Integrated with Azure DevOps
This integration keeps your team informed of activity happening in its Azure DevOps projects. With this integration, code check-ins, pull requests, work item updates, and build events show up directly in your team's Slack channel.
Note: Microsoft Teams would also be a correct answer, but it is not an option here.
References:
https://marketplace.visualstudio.com/items?itemName=ms-vsts.vss-services-slack

Explanation
When you need to pass a secure value (like a password) as a parameter during deployment, you can retrieve the value from an Azure Key Vault. You retrieve the value by referencing the key vault and secret in your parameter file. The value is never exposed because you only reference its key vault ID. The key vault can exist in a different subscription than the resource group you are deploying to.
References:
https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-manager-keyvault-parameter