In this case, I was using the optical out from a WiiM mini into a Yamaha amp. I don't know much about digital audio, but I know that I was able to control the volume of the WiiM's digital output with that setup.
On the other hand, I use a Schiit Asgard at my desk, where I have it connected to my Mac via USB-C. In that setup, I have no control over the volume level going in to the Asgard. MacOS just disables the software volume control when I'm using that audio output.