By default all files are stored on the disk specified as the default_filesystem 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.
toMediaCollection accepts a disk name as a second parameter:
// Will be stored on a disk named s3