Explanation:

Box 1: subscribe
To start monitoring all pipelines in a project, use the following command inside a channel:
@azure pipelines subscribe [project url]
Box 2: https://dev.azure.com/contoso/contoso-app/
Subscribe to a pipeline or all pipelines in a project to receive notifications:
@azure pipelines subscribe [pipeline url/ projecturl]

Explanation:

Project1:Git in Azure Repos
Project2: Github Enterprise
GitHub Enterprise is the on-premises version of GitHub.com. GitHub Enterprise includes the same great set of features as GitHub.com but packaged for running on your organization's local network. All repository data is stored on machines that you control, and access is integrated with your organization's authentication system (LDAP, SAML, or CAS).
Project3: Bitbucket cloud
One downside, however, is that Bitubucket does not include support for SVN but this can be easily amended migrating the SVN repos to Git with tools such as SVN Mirror for Bitbucket .
Note: SVN is a centralized version control system.
References:
https://www.azuredevopslabs.com/labs/azuredevops/yaml/
https://enterprise.github.com/faq

See explanation below
Explanation:
Task 7: Create a Pipeline to Deploy a Docker Image in a New Branch
Step 1: Create a New Branch
* In your Azure DevOps Project (Project1), navigate to Repos.
* In the left menu, click on Branches.
* Click New branch.
* Enter:
* Branch name: azure-pipelines
* Based on: main (or your default branch)
* Click Create.
This creates a new branch named azure-pipelines.
Step 2: Switch to the New Branch
* In Repos, click on Files.
* In the top-left branch selector, choose azure-pipelines.
Step 3: Use the Predefined Docker Pipeline Template
Azure Pipelines has predefined YAML templates for common tasks, including Docker.
Here's how to add it:
* In the azure-pipelines branch, click New file.
* Name the file: azure-pipelines.yml.
* Click the Show templates button in the YAML editor (if available).
* Search for the Docker template in the list of predefined templates.
* Select the Docker template to auto-populate the YAML file.
If the editor doesn't show the template picker, you can manually add the following basic Docker pipeline template:
yaml
Copy
# Predefined Docker pipeline template
trigger:
- main
resources:
- repo: self
variables:
imageName: 'mydockerimage'
stages:
- stage: Build
displayName: Build and push stage
jobs:
- job: Build
pool:
vmImage: 'ubuntu-latest'
steps:
- task: Docker@2
displayName: Build and push an image to container registry
inputs:
command: buildAndPush
repository: $(imageName)
dockerfile: '**/Dockerfile'
containerRegistry: '<your-container-registry-service-connection>'
tags: |
latest
Replace <your-container-registry-service-connection> with your actual Azure Container Registry (ACR) or DockerHub service connection name.
Step 4: Commit the Pipeline to the New Branch
* In the YAML editor, review and adjust if needed (like setting the correct container registry service connection).
* In the bottom commit message box:
* Message: e.g., Add Docker deployment pipeline
* Ensure the branch is set to azure-pipelines.
* Click Commit (not commit to main).
This creates the pipeline YAML file in the new azure-pipelines branch.
Step 5: Create the Pipeline in Azure DevOps
* In the left menu, click Pipelines.
* Click New pipeline.
* Choose:
* Azure Repos Git.
* Select your repo.
* Select Existing Azure Pipelines YAML file.
* In the branch selector, choose the azure-pipelines branch.
* Select the YAML file path (azure-pipelines.yml).
* Click Continue and then Run to validate the pipeline.

see the answer below.
Explanation:
Answer is below

To connect Azure Boards to GitHub.com, connect and configure from Azure Boards. Or, alternatively, install andconfigure the Azure Boards app from GitHub. Both methods have been streamlined and support authenticating and operating via the app rather than an individual.
Note (see step 4 below):
Add a GitHub connection:
Sign into Azure Boards.
Choose (1) Project Settings, choose (2) GitHub connections and then (3) Connect your GitHub account.
If this is your first time connecting to GitHub from Azure Boards, you will be asked to sign in using your GitHub credentials. Choose an account for which you are an administrator for the repositories you want to connect to.
The Add GitHub Repositories dialog automatically displays and selects all GitHub.com repositories for which you are an administrator. Unselect any repositories that you don't want to participate in the integration.

Reference:
https://docs.microsoft.com/en-us/azure/devops/boards/github/connect-to-github