By default all files are stored on the disk specified as the
defaultFilesystem in the config file.
Files can also be stored on any filesystem that is configured in your Laravel app. When adding a file to the media library you can choose on which disk the file should be stored. This is useful when you have a combination of small files that should be stored locally and big files that you want to save on S3.
toMediaLibrary accepts a disk name as a second parameter:
// Will be stored on a disk named s3 $newsItem->addMedia($pathToAFile)->toMediaCollection('images', 's3');