the difference between 0 and unit above 0 on iPhones is about 30% volume.

I have found that when playing audio to a HomePod, pressing Volume Up on the phone increases the volume by 1.

But if you immediately press Volume Down, it goes down by 0.5. So, with two button presses you can get the half-step increase you wanted in the first place.

It's like adding "a little" to a volume change command with Siri.

  "Siri, turn the volume up a little" turns the volume up 0.5.

  "Siri, turn the volume up" turns the volume up one.  

  "Siri, turn the volume up a lot" turns the volume up two.

In macOS, there used to be a modifier key to have the volume change in half-steps, too, but I've forgotten what it is.

I think the only place that Apple has done a good job with volume controls is the AirPods Max. But even there, I'd like more granularity at the low end.