I think you misunderstood @Inspector Gadget, he meant that if the external device (dongle or whatever) contains a DAC then the phone's internal DAC is (or can be) bypassed. And as he explained in his last post, devices can negotiate via USB what is going to happen based on capabilities and settings.
[Edit: sorry I see he also said the phone's DAC is not always present, but never mind, that was not really the heart of the discussion anyway.]
Correct, that is called device enumeration, part of the USB specs
The moment you make a connection the host will ask the slave what its capabilities are.
If I plug my Benchmark DAC1 into my phone, it will report being a 24 bit 44/48/88/96 DAC
@sander99 and @Roseval - If I have an MP3 file which has 16 bit sound at 320 kbps and 44100 Hz. How it will be played differently with external DAC which supports 32 bit at 384 Khz. I do not think it will play any different.