https://www.soundguys.com/usb-audio-explained-18563/
Decent write-up of some of the differences, and quoted below:
For Audio Accessory Mode:
"USB Type-C audio connections don’t actually have to be digital: you can actually get analog audio out too. The USB standard also supports “Audio Accessory Mode”, whereby the D+/D- pins switch roles to support analog audio signals."
And for ADC (Audio Device Class) mode:
"The USB Audio Class specification allows a USB device to identify itself as an audio peripheral. This specification also determines a USB device’s compatibility with a range of specific USB functions and communication types suitable for audio."