RunsOn supports describing various options through a configuration file. This file is located at .github/runs-on.yml (must be named exactly that) in the repository where you are using RunsOn runners.
The configuration file supports defining:
custom images
custom runners
the list of admins having SSH access to the runners
Example
Using the configuration in a GitHub Workflow
Sharing configuration across repositories
RunsOn comes with a feature that allows a local configuration file to inherit from a globally defined configuration file, by using the _extends directive.
The recommendation is to store the global configuration file in the special .github-private repository of your organization, but you can choose any other repository as well (public or private).