Does anyone know what GitHub Actions uses internally for Workflow Orchestration?

Context: Building some workflow orchestration jobs that have similar async needs, and want to build something similar. I love the features that I observe such as concurrency limits, failure handling, etc.

I feel like Argo (https://argoproj.github.io/) is similar, but would love to know if they're using something generally available to the rest of us!

Let me know, Thanks!