This has been public knowledge for years. Github Actions has been based on Azure Devops Pipelines since mid-2019 when it became a full CI/CD solution instead of just performing task automation.

https://docs.github.com/en/actions/reference/specifications-...

> The GitHub-hosted runner application is a fork of the Azure Pipelines Agent.

There was a related discussion on reddit at the time: https://www.reddit.com/r/programming/comments/cnoq0q/github_...

It allowed Github to get spun up quickly with a competitor for Gitlab's CI/CD, and the community was asking for actions to become a CI/CD solution, so taking advantage of pre-existing infrastructure seems like a win-win for everyone. I don't see why they'd bother going to the effort of reinventing the wheel.