I would add to @riban analysis that USB implementation and selection of sources is an area of zynth development, especially audio, that is developed almost by stealth.
The original zynthian take was to treat USB audio as a little bit second rate. The kit included an excellent DtoA so USB was only really used as a test mechanism. It was, for a while, the only way of getting audio in, but even this was used by people and quickly became a must have ( I record stuff with a USB Yeti Mic which makes for a very convenient little rig).
Quite how we allocate USB within the zynthian world as multiple devices start to appear and switching decisions need to be made is something that probably requires some work, but these are success problems so we should be grateful!
The truth is we can do pretty much anything, and if we can lay such developments down so that ‘normal’ users ( We’ve got dungeons full of them … 
) can use the research then so much the better.
I will of course lay in wait for a 
at the end of all this. . .