How do Audio Interfaces work?

I imagine this will be beyond my abilities to DIY for a long while, but I'd like to know how USB audio interfaces work. I'm assuming there's an ADC and then some sort of processor to encode the signal in a USB standard(?) way that DAWs know how to read.

Can anyone correct me if I'm wrong and/or point me to more information about the specs and what sort of hardware and code is usually used in these things?

Many thanks