There's also a 24-bit DSP on-board, which upscales audio to 24-bit, as well as 24-bit DAC.
Fact is you'll be hard pressed finding a DAC installed in a phone during at least the past 5 years that are not accepting 24-bit wording on their bus, with at least 24-bit fixed-point DSP & oversampling, then converting to analog in 24-bit as well (some in 32-bit now as well)
There's no such thing as 24-bit "upscaling"
Or if you wish, everything audio out here already does that with a simple integer multiplication, you just didn't know about it.
But it's just an arithmetic operation, it doesn't improve the quality in any way.
However, does the system preserves this resolution from the audio player API, through the software mixing and processing and eventually to the Linux audio driver, that's something else.
iPhones benefit in dynamic range from 24-bit audio, when playing 24-bit files or even with 16-bit at lower volumes thanks to the volume mixer, but not many Android devices do even since Lollipop that adds 32-bit floating point audio.
Concerning what the marketing material and PR people say, I learned to not trust any of it regarding audio, you can only rely on measurements, they'll say whatever buzzword – sometime in good faith with no correlation whatsoever with reality.
My advice, as usual: measurements 🙂
If you want to know how to do that for your next article: let me know, I'll show you.
HTC One A9 hands-on: A midrange smartphone that feels like a flagship | Ars Technica
A solid, aluminum unibody paired with decent internals and a nice display.