Obviously, if you want to process audio through your mobile device, you need some way to get sound in and out. The most obvious way to do this is to hook up an iOS compatible audio interface. There are countless varieties of these on the market, from affordable and portable options to pro-level interfaces. Check out Focusrite’s iTrack range or IK Multimedia’s iRig devices. Or for something more studiolevel, Arturia’s AudioFuse is iOS ready.
Another option for connecting your device to a DAW is an app like Zerodebug’s Studiomux. This allows users to stream audio and MIDI to/from an iOS device using nothing more than the standard lightning cable.
If you want to keep things in-device, there are plenty of options too. Many apps are now AU-compatible, and can be used as plugins in iOS DAWs such as GarageBand or Cubasis. Beyond that, Audiobus remains a great tool for routing audio in and out of separate apps on your iOS device.