Bluetooth is a codec transport
For a Bluetooth route, the operating system and receiving device negotiate a supported codec and transport configuration. The source file is decoded before that transport step, and the audio may be encoded again for the wireless link. Nyquist therefore does not describe Bluetooth as bit-perfect PCM delivery.
The selected codec, data rate, latency, and processing are not controlled solely by the music player. A Bluetooth device may also apply its own volume, digital signal processing, or sample-rate conversion after reception.
USB exposes device capabilities
A USB DAC advertises the PCM formats and rates it can accept. On macOS, Nyquist can select that CoreAudio device directly, request a compatible nominal rate, inspect the resulting rate, and optionally request exclusive ownership. Compatible hardware can also accept DSD over PCM.
This direct path gives the app more configuration visibility. It does not prove that every USB device is better built or more enjoyable than every wireless device.
Convenience and mobility differ
Bluetooth avoids a cable and usually follows the system route and hardware controls. USB may require an adapter, enough device power, and a suitable cable, especially with iPhone or iPad. On those platforms the operating system still owns more of the route than it does for a directly selected macOS CoreAudio device.
How Nyquist labels the routes
The signal path should identify the active destination and avoid carrying a macOS USB DAC claim over to Bluetooth. Source format remains useful information, but a FLAC source does not make the wireless transport itself lossless or bit-perfect.
Related: using a USB DAC, bit-perfect audio, and AirPlay audio.