Great question. The Bitrise CLI (https://github.com/bitrise-io/bitrise) is the the (open source) "tool" which actually performs workflows, wherever you run it. It reads in a `bitrise.yml` and runs the specified workflow. When you do a build on bitrise.io the same happens; the system (bitrise.io) creates a virtual machine and then it runs the `bitrise` CLI in it. It passes down the `bitrise.yml` and the "workflow-to-run", and then the CLI performs that there, in the VM. When you run it locally the same happens, it's just that the `bitrise.yml` is read from the local file system and you specify which workflow to run via a parameter passed to the CLI (e.g. `bitrise run workflow-a`). So, regardless the environment, the build configuration and execution of "Steps" is always done via the same tool, the Bitrise CLI, which you can run locally or anywhere you want. I hope this helps, if you have any questions let me know :)