An external DAC does the processing for the audio, if you are connecting your phone to the whatever radio you have that would mean that the radio you have is doing the processing so what it outputs depends on that hardware. I would not worry much about the difference in quality, it will be good enough, a car is not made for audiophile level listening since there is so much outside noise there in the first place. Where you would see a difference is if you use a nice dac/amp with the phone along with good headphones.
An external DAC does the processing for the audio, if you are connecting your phone to the whatever radio you have that would mean that the radio you have is doing the processing so what it outputs depends on that hardware.
You are not understanding my replies, if you connect your phone using the port, you will using an external DAC. If it's in the car stereo or not does not mater, I was trying to explain the connection. The iPhone audio stream out through the lightning connector will handle high bit rate music fine. Next is the DAC that converts that to something you can hear, which is why audiophiles buy $100 + DACs and hang them off their phones. If you don't do that, then the iPhone interface in your car will be handling the audio processing, so the quality depends on that. Read this, will explain some and give you options as to what to use https://www.whathifi.com/us/advice/how-to-play-hi-res-music-your-iphone-0
Once you get the car you can play around and see what you like best. No matter what you use it will sound fine, especially if you use a cable vs bluetooth. The simplest solution is plug in the phone to whatever interface the car has and play music. The best sounding solution would be to get a good DAC for the iPhone and run that to the Aux input on the car.