Ok but the iPod could literally act as external storage.

https://en.wikipedia.org/wiki/IPod

> Some versions of the iPod can serve as external data storage devices, like other digital music players. Prior to macOS 10.15, Apple's iTunes software (and other alternative software) could be used to transfer music, photos, videos, games, contact information, e-mail settings, Web bookmarks, and calendars to the devices supporting these features from computers using certain versions of Apple macOS and Microsoft Windows operating systems.

This is / was implemented as a custom protocol to transfer files. It doesn't work out of the box with all programs on any system after the iPod is plugged in - it needs a program (consider it a driver) to be installed on the host computer for it to work. It only works with the programs that speak the protocol - installing the program does make it available to all programs on the host. This is quite different from making the iPod behave like an external hard drive.