I have done very little in the way of production other than use GarageBand a little, but from what I've read, DAWs allow applications to be chained together. But how?
I had pictured they might expose virtual audio devices but that doesn't appear to be the case - is there an open standard or something?
Fundamentally is a DAW just "any software which can do audio processing" or is there more to it than that?
Communication between apps in regards to music happens in two ways:
One way is to use a virtual midi device. In MAC it's built into the OS via Audio Midi Setup -> Midi Studio. In Windows it's via third party apps, "Virtual Midi" is one.
For example, sometimes I write algorithmic music apps in javascript (inside a browser like Chrome) and I want to listen to what I did in Logic. So I set up a virtual midi device on the Mac. and then set it as input in Logic, and I write to it via the browser. With midi you can either write to, or read from a virtual driver. So my Browser is sending information to the DAW that way. But you could do this between DAWs as well.
Another way is to send audio between two apps. This happens on a mac using SoundFlower for example (Blackhole that you mentioned in the comments is an alternative too). So you could be in a DAW and record audio from your computer, for example if you're watching youtube and you want to record the sound, or you want to record the sound from one DAW to another DAW.
So midi is if you want to send notes, or even commands between two midi enabled apps or hardware devices. Audio is if you just need the sound.
