Bluetooth from the phone. It works alright, and even though bluetooth is not the best audio, you will hear the difference between Spotify free v. premium.
Keep in mind if you have a Verizon phone, many plans now include Apple Music for free. And if you have an iPhone, Apple Music works with Siri, so you can just use voice commands while driving, which is great.
Love Spotify, but this bundling of Apple Music on Verizon is going to kill Spotify. (Dropping my premium subscription, because there’s no reason to have both.)