Apple Business Manager allows you to distribute your internal or custom apps to your users without publishing your application on the App Store.
This is useful if you want to deploy your app without making it public. This is also useful if your application to theApple Developer Enterprise Program has been rejected.
To create an ABM account please visit https://business.apple.com/ and register for a new enterprise by filling the form and wait for Apple to confirm your registration.
To distribute your apps, you will also need to register for an Apple Developer Program account here: https://developer.apple.com/programs/enroll/
Once both has been confirmed you can start publishing your apps by following this guide: Publish your iOS apps to ABM