Kinds of USB audio devices
The simplest USB audio device is a USB microphone, which can be plugged direct into your computer and used to record.
These are popular with podcasters, for example. At the other end of the list is something like a mixing console, which will have many inputs, allowing your to hook up, say, 16 different inputs and have them all piped into your GarageBand app.
In between there’s a huge range of options, from USB headphone amps costing many thousands of dollars, and meant for just one pair of headphones, to budget mixers that let musicians record great quality audio at home on modest budgets.