About visibility for GitHub Apps
A GitHub App can be public, private, or internal. If you set your GitHub App registration to public, anyone on your GitHub Enterprise Server instance can install it, but the app is not available outside of your instance. If you set your GitHub App registration to private, it can only be installed on the account that owns the app.
GitHub Apps owned by an enterprise account have "internal" visibility. Internal apps can only be installed by organizations within the enterprise and can only be authorized by users within the enterprise. Members of the enterprise and unaffiliated users can authorize these apps, but outside collaborators cannot.
If you want your GitHub App to be available to organizations in a GitHub Enterprise Server instance that you are not part of, then you need to take additional steps. For more information, see Making your GitHub App available for GitHub Enterprise Server.
If it is important for other GitHub Enterprise Server users to be able to use your tool, consider using GitHub Actions instead of a GitHub App. Public actions are available on GitHub Enterprise Server instances with GitHub Connect. For more information, see Enabling automatic access to GitHub.com actions using GitHub Connect and About GitHub Actions for enterprises.
For information about changing the visibility of a GitHub App registration, see Modifying a GitHub App registration.
Public installation flow
Public GitHub Apps have a landing page with an Install button, so that other people can install the app on their accounts.
Private installation flow
Private GitHub Apps can only be installed on the user or organization account of the app owner. Limited information about the app will exist on a landing page for the app, and the Install button will only be available to organization owners and app managers for the organization that owns the app, or the personal account if the GitHub App is owned by an individual account.
"Internal" installation flow
Enterprise-owned GitHub Apps can only be installed on organizations within the enterprise by using the installation URL. The app can not be installed on user accounts.