And yeah, there are apps like qjackctl and Carla and others which do let you do this kind of mixing. But yeah, I'm talking about the basic sound settings that most users are going to instinctually mess with and make quick changes. As mentioned, there are software stacks that will let you mix inputs and outputs on Windows using synthetic devices, but that's not the natively exposed tools present on a standard install. On most modern Linux distros, you could get it done with some fancy CLI work out of the box, or you'll want to install tools like qjackctl.
