The latest iPhone doesn't have a DAC in it for headphones. The little 3.5mm-to-lightning dongle is active -- it contains a tiny DAC/amp powered by the lightning port.
Is the tiny lightning dongle a good DAC? The answer is largely "it's fine". Anyone who isn't on the cork-sniffing side of audiophile trends will find it acceptable, assuming the amp is capable of driving the load of whatever headphones you're plugging into it.
https://www.audiosciencereview.com/forum/index.php?threads/r...
I remember when I had a first gen iPhone and the call quality was atrocious with AT&T compared to my RAZR on Verizon. You get used to it over time but back to back it was pretty dramatic.
https://www.whathifi.com/advice/how-to-play-hi-res-music-you...
Quoting the article:
"Next, you need a decent DAC such as the Audioquest DragonFly Cobalt, Cyrus soundKey or Chord Mojo 2 and a good pair of headphones, such as the Grado SR325x, Shure Aonic 3 or Austrian Audio Hi-X55.
This is just a starting point, of course. Don’t be afraid to build up to a more revealing system. You could combine the Chord Hugo 2 DAC with a pair of Beyerdynamic T1 (3rd Generation) headphones for a more premium set-up. That might be a touch overkill and punishingly transparent for an iPhone, but these are hi-res files, after all."
More to the point I don't want dongles for my phone either. These are all solutions for Apple cheapening the product.
I’ve got a decent quality pro audio interface and monitor headphones. I’ve also got airpod pros. The AirPods sound much much better and I prefer listening to anything with them.
It’s all subjective.