I have Premium Subscription. How can I download Spotify music to computer from iPhone?

How to download Spotify music to computer from iPhone：

• You can install the Spotify app on your computer and log in with the same account. You'll have access to all your playlists, and you can stream music directly from your computer.
• If you want MP3 files specifically for your car and want to listen offline, you may purchase music from digital stores (like iTunes Store) or convert your owned files to MP3 format using a legitimate conversion tool (but not Spotify music).
• If your car has a CD player, consider burning your Spotify playlists to a CD as a way to create physical copies. However, this is more of a workaround and might not be the most convenient option.

doubt it's possible really

but you can of cause on said computer likely install the spotify app and download songs from that to the computer

You may not know, Spotify Premium allows you to download music to computer offline, but these files are encrypted and cannot be directly converted to MP3. However, you can try using the Spotify client on your computer, log in to the same account, and then download and sync the songs to your car system offline through your computer.

If you are using a Mac, then AirDrop the downloaded Spotify music file to your Mac. After that, use the free audio conversion app such as FFMPEG:

for i in *.ogg; do ffmpeg -i "$i" "${i%.*}.mp3"; done

The above command will batch convert Spotify ogg to mp3 in bulk.