[Essential Phone Accessories] firefly6240:
This might shed some light on things.

There are two types of USB c dongles (and in turn headphones since the USB c side is essentially a built in adaptor)

The first type is basically just an analog pass through and functions like a standard headphones Jack just using a USB c end instead of the standard headphones plug.

These adaptors do not have a built in DAC and rely on the phones internal DAC for processing just as a regular headphones Jack would.

These are often labeled on Amazon as for moto z only since that phone supports this mode. (Though other phones support it as well)

These adaptors and headphones are usually much cheaper because they don't have a DAC.

These won't work with the essential phone.

The second type has a built in DAC.

The essential adaptor is of this type as is the Google pixel and Razer phone adaptors.

The essential phone will only work with adaptors that have a built in DAC.

It will also work in the exact same fashion with a dedicated external DAC, for example I use an audioquest dragonfly black with a USB C to A adaptor.

Now for the USB audio player pro.

What this does is uses it's own driver to bypass the standard way Android handles audio.

This works very well but ONLY works in the USB audio player pro app for listening to music. (You couldn't use the USB audio player pro driver to watch videos or play games for example)

It also would not enable the first type of adaptor (analog) to work with the essential phone.

It basically unlocks the full potential of your DAC for listening to music within the app.

The first benefit here is that Android by default locks all DACs to a 48k sample rate. ( This is true for both internal and external DACs)

Some DACs are capable of many other modes. (My dragonfly allows for 96k for studio tracks or 44k for Redbook audio CD format for example)

All audio will sound better at it's native sample rate.

Most music sounds best at 44k 16 bit because it usually comes from a recording meant for a CD.

Studio tracks might be 24 bit 96k and would sound best at that sample rate.

Things like games and movies do sound better at native rates but aren't as noticable when re-sampled at a different rate.

The second benefit is USB audio player pro supports hardware volume if your DAC has it.

You might have noticed that the volume from most adaptors has a much lower volume than other phones.

This is because of the software volume limits.

My dragonfly for example has a built in headphones amp.

When using USB audio player pro the volume is at least double what it is through Android natively and sounds impressively cleaner because of the better signal to noise ratio.

It should be noted that even using a high end DAC, when not using USB audio player pro or some other app that handles the audio driver directly you will be limited to 48k with software volume control.

And to get the benefits of USB audio player pros driver you have to have the DAC in exclusive mode, meaning it's locked to that app until unplugged it otherwise reinitialized.

When you first plug in your DAC Android will ask if you want to use it with USB audio player pro (or similar app if installed) if you tap ok audio will only come from that app.

So if you open a game you would have no audio from the game until you unplug and replug the adaptor for example.

If you choose cancel, then it defaults to Android and locks the DAC to 48k with software volume but works in all apps.

I know that was really long winded but hopefully its helpful to someone.