I would guess these days, many people buy audiophile DACs for the amplifier stage, for use with demanding high impedance headphones. However, there have been some really bad phone DACs in the past, the Qualcomm audio from the Note4 that I used to own sounded significantly worse than the iPhone using IEMs. What are people measuring with regard to the Apple dongle? Is it "objectively superior" with regard to multiple measurements?

It shouldn't be too hard to build a decent DAC and amp stage, at least for ear buds and IEMs, it's just that many manufacturers (e.g. Samsung / Qualcomm) don't care. Apple sell music and so probably should care more.