Airfoil is worth every penny of its $29. It will stream anything from your Mac to any Sonos, Chromecast, Bluetooth, or Airplay device … including the HomePod.
Here is the Airfoil window when I stream from the InstaRadio Mac app:
That’s an old blog post from 2012; it’s possible that the procedure has been updated but I don’t know - pretty much everything I’ve wanted to stream from iOS to my HomePod (and to my powered monitors connected to an Airport Express elsewhere in my space) has been streamable directly through AirPlay.
At any rate, Airfoil is a Mac app and not an iOS app, though you can make an iOS device into a virtual speaker itself, streaming anything from a Mac or Windows with the accompanying Airfoil Satellite app.