• First it's important to understand iDAM. iDAM allows you to connect iOS devices to a Mac using a USB cable, then to route audio from the iOS device to the Mac (but not the other way around), and to route MIDI to and from both devices. A Google search for “Apple iDAM” should get you several videos and articles. The one I found the most helpful is here: https://www.cultofmac.com/615742/how-to-record-digital-audio-from-your-iphone-to-your-mac-with-idam/.
• With iDAM you'll find that you can route MIDI from the iPad to the Mac, and out from apps like Ableton Live and GarageBand without any problem. You can also route a MIDI controller attached to the iOS device through iDAM to the Mac. However, hook that same device up to the Mac and you'll find that you can't route the external controller back to iOS. The reason for this is beyond the scope of this article.
• There is a free MacOS utility called MidiPipe that can set up a route. You'll need to download and install this app from http://www.subtlesoft.square7.net/MidiPipe.html.

1. Run MidiPipe. You'll need to keep this running any time you want to do this routing.
2. Double-click MIDI In in the Tools list so that it shows up in the Pipes list. (Drag and drop will work too.)
3. Highlight the MIDI In pipe and select your MIDI controller from the dropdown list under MIDI Input
4. Double-click MIDI Out in the Tools list so that it shows up in the Pipes list.
5. Highlight the MIDI Out pipe and select your iOS device from the dropdown list under MIDI Output.
6. If you have other devices connected to the Mac, set up MIDI In pipes for each. Be sure to drag them above the MIDI Out node as flow is from top to bottom.
7. On iOS test that the external controllers send MIDI over the iDAM MIDI Host connection.
8. Use File > Save in MIDI Pipe for easy recall of this setup.

There are many other things you can do with MidiPipe. It's well worth looking into, and highly encouraged to make a donation to the developer for their generous work.