Given that Apple are selling iPhones with 1TB of storage these days, you'd expect them to test the ability to import huge libraries. (even if the 13 Pro still uses USB 2.0 so the transfer speeds would be abysmal)
I don't think it was using iCloud. It would show up in iTunes (haven't used my Mac much since iTunes got split up) and I could drag files around from some applications and back it up to my local drive (in addition to the iCloud backups).