While much of what you're asking about is subjective, I'll answer the main thing going on here, which is what's going on with the USB port in the back. The simple answer is, if there's a rear USB port, the manufacturer will most likely include the USB extension with the head unit. You can place the extension to any place you want to. I've found it very easy to place it at some point where the dash molding meets the body with just the end sticking out. Some place inconspicuous which allows easy access. I've never drilled a hole for it as it wasn't needed. You run the cable during installation with the dash molding removed. It is fairly easy.

From there, I've usually put a thumb drive with my music on it. You could also (I'd assume) attach any other USB enabled device, like an iPhone or Android phone, using the port. Sky's the limits ... whatever your head unit can deal with. This isn't a hard process ... you just have to plan where you're going to have the USB cable hanging out. It could be completely hidden, or completely obvious. This would be your choice.