When getUrl is called, the task of generating that URL is passed to an implementation of Spatie\MediaLibrary\Support\UrlGenerator\UrlGenerator.
The package contains a DefaultUrlGenerator that can generate URLs for a media library that is stored inside the public path. It can also generate URLs for S3.
If you are storing your media files in a private directory or are using a different filesystem, you can write your own UrlGenerator. Your generator must implement the Spatie\MediaLibrary\Support\UrlGenerator\UrlGenerator interface.