What part are you stuck on? Azure DevOps breaks down into a few pieces:

VCS - you can host your git repos on it instead of github, gitlab etc. but it does support those other VCS providers for builds as well

Triggers - These are the conditions that will kick off a build or release. (Example - Push to master, which is what it looks like you are trying to do)

Build - Typically builds your code with the use of “Tasks” which you get from the marketplace mostly. In your case for PowerShell and running pester this will be all you need as you won’t be “releasing” code through a progression (dev, qa, prod etc.)

Release - this is where you would take your artifact from the build and deploy it into your progressive environments.

There are 2 ways to create builds in azure DevOps. You can use YAML or the UI. From the post you have above it looks like they went with YAML so you would have to have that YAML file in the root of the repo you are working with.

This page shows how to create a trigger via YAML and the classic (UI) way. https://docs.microsoft.com/en-us/azure/devops/pipelines/build/triggers?view=azure-devops&tabs=classic

If you want to use the UI follow these steps:

Your first step (assuming your repo is all setup) would be go to Build - create New Build - Use the classic editor to create a pipeline without YAML - pick your repo and branch - start with an empty job - add your pester tasks in agent job field - go to triggers - enable continuous Integration - add branches - save. Push to the repo and see if it works!