> GitHub Actions is based on Visual Studio Team Foundation Server's CI, and later Azure DevOps
Yes and no, ADO Agent (https://github.com/microsoft/azure-pipelines-agent) is far more secretive and "black-box" alike. Like stuck in old version of NodeJS, Powershell, API without documentation or even enough tests/samples...
You can also compare the source code structure to the Azure Devops Pipeline agent's to tell pretty easily that the github runner is a fork what has been edited to process the somewhat different format of the actions YAML.
Its forked version of AzureDevOps Agent